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

http://projects.theforeman.org/issues/5503
https://bugzilla.redhat.com/show_bug.cgi?id=1102315
They all sound a little strange, but the one that is most clear probably is best. We were trying to avoid things like `allowedTo` and `notAllowedTo` ? or `ableTo`/`notAbleTo` ? Best other ideas I got off the top of my head.