Add conf_enable_app to add items to $enabled_apps, used from conf_enable_xxx functions so one doesn't need to both add stuff to $eanbled_apps and then call conf_enable_xxx, and to avoid enabling the same app twice.

New conf_enable_wiki function to follow the interface of other apps.

apps/bridge/app.rc

@@ -7,6 +7,7 @@ fn conf_enable_comments {
     }
     enable_comments=yes
     groups_allowed_comments=$*
+    conf_enable_app bridge
 }
 
 fn bridge_init {