Custom loadout sanity (#2671)

This PR unifies the various manual implementations of custom loadout equip into three procs located inside the job subsystem, as well as updates all custom loadout code to use the new procs.

Fixes custom loadout items not overriding job items in character setup.

code/modules/mob/new_player/preferences_setup.dm

@@ -216,29 +216,16 @@ datum/preferences
 
 	if(previewJob)
 		mannequin.job = previewJob.title
+
+		var/list/leftovers = list()
+		var/list/used_slots = list()
+
+		SSjobs.EquipCustom(mannequin, previewJob, src, leftovers, null, used_slots)
+
 		previewJob.equip_preview(mannequin, player_alt_titles[previewJob.title])
-		var/list/equipped_slots = list() //If more than one item takes the same slot only spawn the first
-		for(var/thing in gear)
-			var/datum/gear/G = gear_datums[thing]
-			if(G)
-				var/permitted = 0
-				if(G.allowed_roles)
-					for(var/job_name in G.allowed_roles)
-						if(previewJob.title == job_name)
-							permitted = 1
-				else
-					permitted = 1
 
-				if(G.whitelisted && (G.whitelisted != mannequin.species.name))
-					permitted = 0
+		SSjobs.EquipCustomDeferred(mannequin, src, leftovers, used_slots)
 
-				if(!permitted)
-					continue
-
-				if(G.slot && !(G.slot in equipped_slots))
-					equipped_slots += G.slot
-					var/metadata = gear[G.display_name]
-					mannequin.equip_to_slot_or_del(G.spawn_item(mannequin, metadata), G.slot)
 		mannequin.update_icons()
 
 /datum/preferences/proc/update_preview_icon()