Adds event regions to GameMaster events (#8334)

* Configures gm events to be selectable by "region"

* Configures event regions for existing events

* Configures event selection to filter by region

code/_helpers/_lists.dm

@@ -238,19 +238,17 @@ This actually tests if they have the same entries and values.
 //Pretends to pick an element based on its weight but really just seems to pick a random element.
 /proc/pickweight(list/L)
 	var/total = 0
-	var/item
-	for (item in L)
-		if (!L[item])
+	for(var/item in L)
+		if(!L[item])
 			L[item] = 1
 		total += L[item]
 
 	total = rand(1, total)
-	for (item in L)
-		total -=L [item]
-		if (total <= 0)
+	for(var/item in L)
+		total -= L[item]
+		if(total <= 0)
 			return item
 
-	return null
 
 //Pick a random element from the list and remove it from the list.
 /proc/pick_n_take(list/listfrom)