Adds forum-based link config setting

code/controllers/configuration.dm

@@ -102,6 +102,7 @@
 	var/donationsurl = "http://example.org"
 	var/repositoryurl = "http://example.org"
 	var/discordurl = "http://example.org"
+	var/discordforumurl = "http://example.org"
 
 	var/overflow_server_url
 	var/forbid_singulo_possession = 0
@@ -487,6 +488,9 @@
 				if("discordurl")
 					config.discordurl = value
 
+				if("discordforumurl")
+					config.discordforumurl = value
+
 				if("donationsurl")
 					config.donationsurl = value