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.
It's kind of nice to know total errata if you only care about how out of date you are.  And if you do care about that then you don't have to do any addition to get there.  I'm not opposed to removing it though.