Fixes #5503/BZ1102315 - restrict UI interactions to actual permissions.

http://projects.theforeman.org/issues/5503
https://bugzilla.redhat.com/show_bug.cgi?id=1102315
Yeah, I was thinking that after adding the 352462th one.  Maybe `permissionDenied()`?  Or maybe `withPermission()` and `withoutPermission()`?

Definitely open to suggestions on the naming.