Adds the ability for ERTs to use a custom shuttle (#71348)

## About The Pull Request
ERTs can now have a custom shuttle template defined on their type,
which, should the "use custom shuttle" toggle be enabled while creating
the ERT, will spawn them in deep space on the given shuttle instead of
at Centcom.

Applies the new system to the bounty hunter ERT

## Why It's Good For The Game
Not all ERTs are NT enough to start at Centcom all the time, and it
gives more control over ERTs

## Changelog
🆑
admin: ERTs can now have a custom shuttle that they can spawn on, on a
toggle while creating said ERT.
/🆑

code/modules/admin/verbs/ert.dm

@@ -21,6 +21,7 @@
 	.["mainsettings"]["leader_experience"]["value"] = newtemplate.leader_experience ? "Yes" : "No"
 	.["mainsettings"]["random_names"]["value"] = newtemplate.random_names ? "Yes" : "No"
 	.["mainsettings"]["spawn_admin"]["value"] = newtemplate.spawn_admin ? "Yes" : "No"
+	.["mainsettings"]["use_custom_shuttle"]["value"] = newtemplate.use_custom_shuttle ? "Yes" : "No"
 
 
 /datum/admins/proc/equipAntagOnDummy(mob/living/carbon/human/dummy/mannequin, datum/antagonist/antag)
@@ -87,11 +88,12 @@
 		"open_armory" = list("desc" = "Open armory doors", "type" = "boolean", "value" = "[(ertemplate.opendoors ? "Yes" : "No")]"),
 		"leader_experience" = list("desc" = "Pick an experienced leader", "type" = "boolean", "value" = "[(ertemplate.leader_experience ? "Yes" : "No")]"),
 		"random_names" = list("desc" = "Randomize names", "type" = "boolean", "value" = "[(ertemplate.random_names ? "Yes" : "No")]"),
-		"spawn_admin" = list("desc" = "Spawn yourself as briefing officer", "type" = "boolean", "value" = "[(ertemplate.spawn_admin ? "Yes" : "No")]")
+		"spawn_admin" = list("desc" = "Spawn yourself as briefing officer", "type" = "boolean", "value" = "[(ertemplate.spawn_admin ? "Yes" : "No")]"),
+		"use_custom_shuttle" = list("desc" = "Use the ERT's custom shuttle (if it has one)", "type" = "boolean", "value" = "[(ertemplate.use_custom_shuttle ? "Yes" : "No")]"),
 		)
 	)
 
-	var/list/prefreturn = presentpreflikepicker(usr,"Customize ERT", "Customize ERT", Button1="Ok", width = 600, StealFocus = 1,Timeout = 0, settings=settings)
+	var/list/prefreturn = presentpreflikepicker(usr, "Customize ERT", "Customize ERT", Button1="Ok", width = 600, StealFocus = 1,Timeout = 0, settings=settings)
 
 	if (isnull(prefreturn))
 		return FALSE
@@ -109,36 +111,75 @@
 		ertemplate.teamsize = prefs["teamsize"]["value"]
 		ertemplate.mission = prefs["mission"]["value"]
 		ertemplate.polldesc = prefs["polldesc"]["value"]
-		ertemplate.enforce_human = prefs["enforce_human"]["value"] == "Yes" // these next 5 are effectively toggles
+		ertemplate.enforce_human = prefs["enforce_human"]["value"] == "Yes" // these next 6 are effectively toggles
 		ertemplate.opendoors = prefs["open_armory"]["value"] == "Yes"
 		ertemplate.leader_experience = prefs["leader_experience"]["value"] == "Yes"
 		ertemplate.random_names = prefs["random_names"]["value"] == "Yes"
 		ertemplate.spawn_admin = prefs["spawn_admin"]["value"] == "Yes"
+		ertemplate.use_custom_shuttle = prefs["use_custom_shuttle"]["value"] == "Yes"
 
 		var/list/spawnpoints = GLOB.emergencyresponseteamspawn
 		var/index = 0
 
+		var/list/mob/dead/observer/candidates = poll_ghost_candidates("Do you wish to be considered for [ertemplate.polldesc]?", "deathsquad")
+		var/teamSpawned = FALSE
+
+		// This list will take priority over spawnpoints if not empty
+		var/list/spawn_turfs = list()
+
+		// Takes precedence over spawnpoints[1] if not null
+		var/turf/brief_spawn
+
+		if(!length(candidates))
+			return FALSE
+
+		if(ertemplate.use_custom_shuttle && ertemplate.ert_template)
+			to_chat(usr, span_boldnotice("Attempting to spawn ERT custom shuttle, this may take a few seconds..."))
+			var/datum/map_template/shuttle/ship = new ertemplate.ert_template
+			var/x = rand(TRANSITIONEDGE, world.maxx - TRANSITIONEDGE - ship.width)
+			var/y = rand(TRANSITIONEDGE, world.maxy - TRANSITIONEDGE - ship.height)
+			var/z = SSmapping.empty_space.z_value
+			var/turf/located_turf = locate(x, y, z)
+			if(!located_turf)
+				CRASH("ERT shuttle found no place to load in")
+
+			if(!ship.load(located_turf))
+				CRASH("Loading ERT shuttle failed!")
+
+			var/list/shuttle_turfs = ship.get_affected_turfs(located_turf)
+
+			for(var/turf/affected_turf as anything in shuttle_turfs)
+				for(var/obj/effect/landmark/ert_shuttle_spawn/spawner in affected_turf)
+					spawn_turfs += get_turf(spawner)
+
+				if(!brief_spawn)
+					brief_spawn = locate(/obj/effect/landmark/ert_shuttle_brief_spawn) in affected_turf
+
+			if(!length(spawn_turfs))
+				stack_trace("ERT shuttle loaded but found no spawnpoints, placing the ERT at wherever inside the shuttle instead.")
+
+				for(var/turf/open/floor/open_turf in shuttle_turfs)
+					if(!is_safe_turf(open_turf))
+						continue
+					spawn_turfs += open_turf
+
+
 		if(ertemplate.spawn_admin)
 			if(isobserver(usr))
-				var/mob/living/carbon/human/admin_officer = new (spawnpoints[1])
+				var/mob/living/carbon/human/admin_officer = new (brief_spawn || spawnpoints[1])
 				var/chosen_outfit = usr.client?.prefs?.read_preference(/datum/preference/choiced/brief_outfit)
 				usr.client.prefs.safe_transfer_prefs_to(admin_officer, is_antag = TRUE)
 				admin_officer.equipOutfit(chosen_outfit)
 				admin_officer.key = usr.key
+
 			else
 				to_chat(usr, span_warning("Could not spawn you in as briefing officer as you are not a ghost!"))
 
-		var/list/mob/dead/observer/candidates = poll_ghost_candidates("Do you wish to be considered for [ertemplate.polldesc]?", "deathsquad")
-		var/teamSpawned = FALSE
-
-		if(candidates.len == 0)
-			return FALSE
-
 		//Pick the (un)lucky players
-		var/numagents = min(ertemplate.teamsize,candidates.len)
+		var/numagents = min(ertemplate.teamsize, length(candidates))
 
 		//Create team
-		var/datum/team/ert/ert_team = new ertemplate.team ()
+		var/datum/team/ert/ert_team = new ertemplate.team()
 		if(ertemplate.rename_team)
 			ert_team.name = ertemplate.rename_team
 
@@ -167,9 +208,14 @@
 			earmarked_leader = pick(candidates)
 
 		while(numagents && candidates.len)
-			var/spawnloc = spawnpoints[index+1]
-			//loop through spawnpoints one at a time
-			index = (index + 1) % spawnpoints.len
+			var/turf/spawnloc
+			if(length(spawn_turfs))
+				spawnloc = pick(spawn_turfs)
+			else
+				spawnloc = spawnpoints[index+1]
+				//loop through spawnpoints one at a time
+				index = (index + 1) % spawnpoints.len
+
 			var/mob/dead/observer/chosen_candidate = earmarked_leader || pick(candidates) // this way we make sure that our leader gets chosen
 			candidates -= chosen_candidate
 			if(!chosen_candidate?.key)