Add admin flag to users API
Currently, it is not possible to distinguish admin users from "normal" users via the API. This patch adds the
admin flag to the users API. The flag is added to the index action (visible only to admins) and on the show action to admins only.
Note that due to a peculiarity of the API builder, the field is only included in JSON responses if the value is
true. For XML, it is included with
false values respectively. With the JSON API, it is thus not possible to distinguish a list of non-admin users from a list of users without the permission to see the admin status.