user_groups: Add can_mention_group_id field to UserGroup objects.

This commit adds code to include can_mention_group_id field to
UserGroup objects passed with response of various endpoints
including "/register" endpoint and also in the group object
send with user group creation event.

Fixes a part of #25927.

web/tests/lib/events.js

@@ -795,6 +795,7 @@ exports.fixtures = {
             members: [1],
             is_system_group: false,
             direct_subgroup_ids: [2],
+            can_mention_group_id: 11,
         },
     },