Expose `gitlab_employee?` to issue/MR assignee component
frontend counterpart: #216649 (closed)
The following endpoints/serializers should be updated to include the is_gitlab_employee
property on the user object.
/issues/{issue_id}.json?serializer=sidebar_extras
- The value of this line
/autocomplete/users.json
/merge_requests/{mr_id}.json?serializer=sidebar_extras