Allow groups to be used as aliases for user mention

when configured by the admin a group can be found through the @mentions
feature in both the compose/reply and the private message user-selectors
and once selected the mention will be replaced by the list of users in
the group

spec/controllers/admin/groups_controller_spec.rb

@@ -21,7 +21,8 @@ describe Admin::GroupsController do
       "id"=>group.id,
       "name"=>group.name,
       "user_count"=>1,
-      "automatic"=>false
+      "automatic"=>false,
+      "alias_level"=>0
     }]
   end