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

app/serializers/basic_group_serializer.rb

@@ -1,3 +1,3 @@
 class BasicGroupSerializer < ApplicationSerializer
-  attributes :id, :automatic, :name, :user_count
+  attributes :id, :automatic, :name, :user_count, :alias_level
 end