admin flag accessible to admins

Setting the api flag in the /users POST / PUT api is already possible. This PR adds the is_admin flag to the attributes accessible to admins so it can also be fetched via the /users GET api.

Feedback post: http://feedback.gitlab.com/forums/176466-general/suggestions/4529842-allow-admin-to-view-and-set-admin-flag-on-users-by