Fixes #6468 - Allow filtering/searching by active user

lgtm except that 'current_user' appears at the bottom of the list. Maybe it's better at the top of the list or in alphabetical order. Also, the autocomplete doesn't work when typing 'cu..." However, when typing in something else (ex. bob ...) then 'current_user' still appears in the list, and I'm not sure it should. I feel ok to merge as is and create these as a separate issue. http://projects.theforeman.org/issues/7440