Limits the ability of the SM to return to the station zlevel once ejected.

Fixes #7900

code/controllers/configuration.dm

@@ -172,6 +172,7 @@ var/list/gamemode_cache = list()
 	var/list/admin_levels= list(2)					// Defines which Z-levels which are for admin functionality, for example including such areas as Central Command and the Syndicate Shuttle
 	var/list/contact_levels = list(1, 5)			// Defines which Z-levels which, for example, a Code Red announcement may affect
 	var/list/player_levels = list(1, 3, 4, 5, 6)	// Defines all Z-levels a character can typically reach
+	var/list/sealed_levels = list() 				// Defines levels that do not allow random transit at the edges.
 
 	// Event settings
 	var/expected_round_length = 3 * 60 * 60 * 10 // 3 hours