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

Add Errata counts to the Content hHost list and refactor into a
directive.  Also update usage in Content View Versions.

http://projects.theforeman.org/issues/7704

Note that these totals will always show 0 until http://projects.theforeman.org/issues/7705 is completed.
We decided to pull out the ____ Errata () part because it is useful for the content view versions page but not for the systems page.