Move abstract model to katello-common package

It's being used in both Katello and Headpin mode (at least form
app/lib/notifications/notifier.rb).
Worked on content view search
Content Search: created a content view comparison
Content Search: Refactored content view comparison

Refactored content view comparison by breaking out code into classes
Content Search: refactored code by creating module namespaces
Content Search: Fixed metadata row in view comparison
Content search: fixed link and removed duplicate code
Content search: added pagination to content view comparison
Content search: Fixing product_repos method name
Content Search: moving files to app/lib
Content Search: not displaying total packages per product on cv comparison
Content search: fixed search modes for cv comparison
Content Search: tweaked product row in cv comparison
Content Search: showing cv filter for package search
Content Search: fixes to existing code

Filtering product and repo searches by default content views and added a new
ProductRow model.
Content Search: filtering views by search mode
Merge pull request #1752 from daviddavis/cvcs

Content search: worked on content view search and fixed existing searches
content views - initial work to show CVs on product search
Merge branch 'default_view_change' of github.com:jlsherrill/katello into content_search
Merge branch 'default_view_change' of github.com:jlsherrill/katello into content_search