fixes #6488 - enable autosearch completion on Katello model filters

First shot at this, I've got some questions:

1) Should the AutoCompleteSearch go through the API controllers instead? I've had to create normal controllers for the models to handle the requests to keep the Foreman style of URL's.  Although with theforeman/foreman#1452 we can route it wherever.

2)  app/controllers/katello/auto_complete_search.rb  - this isn't significantly different from Foreman's, could we use theirs instead?  The only difference is 'filter' feature, but I don't see it used anywhere...? Also shouldn't it be a concern?

3) What are all these auto_complete methods everywhere? Intended for the angular views it seems? Shouldn't we try to use the same style as Foreman for searching?

The idea here is so that permissions can be evaluated against an arbitrary target defined by the controller -- e.g. content_hosts#auto_complete_search, not auto_complete_search#auto_complete_search.