Adds the ability for factions to blacklist specific citizenships (#21129)

Nothing player-facing in this one, for now. This adds the ability for
factions to be unavailable for certain citizenships.
It has been implemented as a blacklist due to the variety of
citizenships in the game. An example would be if the DPRA was added to
the list for Idris, then anybody with a DPRA citizenship would be unable
to select Idris as their faction.
This was done for the taj lore team.

---------

Signed-off-by: Casper3667 <8396443+Casper3667@users.noreply.github.com>
Co-authored-by: SleepyGemmy <99297919+SleepyGemmy@users.noreply.github.com>

code/game/jobs/faction/faction.dm

@@ -7,6 +7,7 @@
 	var/list/allowed_role_types
 	var/list/allowed_species_types
 	var/list/job_species_blacklist //will override the normal job species list for a member of this faction
+	var/list/blacklisted_citizenship_types
 
 	var/is_default = FALSE
 
@@ -23,6 +24,16 @@
 
 	allowed_species_types = l
 
+	var/list/b = list()
+
+	for(var/path in blacklisted_citizenship_types)
+		if(blacklisted_citizenship_types[path] != TRUE)
+			b |= typecacheof(path, FALSE)
+		else
+			b[path] = TRUE
+
+	blacklisted_citizenship_types = b
+
 /datum/faction/proc/get_occupations()
 	. = list()
 
@@ -50,16 +61,25 @@
 			. += role
 
 /datum/faction/proc/get_selection_error(datum/preferences/prefs, var/mob/user)
-	if (!length(allowed_species_types))
-		return null
+	if(length(allowed_species_types))
 
-	var/datum/species/S = prefs.get_species_datum()
+		var/datum/species/S = prefs.get_species_datum()
 
-	if (!S)
-		return "No valid species selected."
+		if(!S)
+			return "No valid species selected."
 
-	if (!is_type_in_typecache(S, allowed_species_types))
-		return "Invalid species selected."
+		if(!is_type_in_typecache(S, allowed_species_types))
+			return "Invalid species selected."
+
+	if(length(blacklisted_citizenship_types))
+
+		var/datum/citizenship/C = SSrecords.citizenships[prefs.citizenship]
+
+		if(!C)
+			return "No valid nation selected."
+
+		if(is_type_in_typecache(C, blacklisted_citizenship_types))
+			return "Invalid nation selected."
 
 	if (!is_visible(user))
 		return "This faction is not available to you."