Fixes #6870/BZ1075523: use ANONYMOUS_ADMIN for hidden user.

Foreman recently added the concept of an ANONYMOUS_ADMIN to User.
This commit makes use of that in order to ensure that this user
is not displayed on the users page.

http://projects.theforeman.org/issues/6870
https://bugzilla.redhat.com/show_bug.cgi?id=1075523
Just to reduce the blast radius since this is a bug fix slated for a future release.  I'm open to using  `User.anonymous_admin` directly instead if you insist.