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.

zerver/lib/event_schema.py

@@ -1693,6 +1693,7 @@ group_type = DictType(
         ("direct_subgroup_ids", ListType(int)),
         ("description", str),
         ("is_system_group", bool),
+        ("can_mention_group_id", int),
     ]
 )