Fixes #7704: show applicable Errata count for Content Host list.

Add Errata counts to the Content hHost list and refactor into a
directive.  Also updates usage in Content View Versions.

http://projects.theforeman.org/issues/7704
Merge pull request #4698 from waldenraines/errata-management

Fixes #7704: show applicable Errata count for Content Host list.