Fixes #7953: add product/repositories list for an erratum.

Display a list of products/repositories that an erratum is in.

http://projects.theforeman.org/issues/7953
This change caused the errata list page to go from taking .3 seconds -> 15 seconds on my dev environment.  If its not much much improved, you may consider using the repositories controller to fetch the list of repositories.  (i'd be a proponent of that regardless)