Feature #3914 Set taxonomies from user defaults on login

This PR allows users to have default taxonomies. These can be set on the users/edit form, although something like what @mccun934‎ suggested (http://projects.theforeman.org/attachments/download/654/org-selector-fav.png) would be nice to have in the future. I preferred to wait until inheritable taxonomies (https://github.com/theforeman/foreman/pull/1105) gets merged so that this can go in 1.4.

Also, I removed the restriction that only allowed admin users to clear their taxonomy selection. I think there's no reason to only let admins do that, not to mention that regular users could do this before this PR through the URL directly, this just exposes the option in the UI.

http://projects.theforeman.org/issues/3914