Content Views Rework: APIs for available/current content

This commit contains support for several additional APIs
to support the content view rework.  This includes:

ContentView - GET puppet_modules & available_puppet_modules
Filter - GET package_groups & available_package_groups
         GET errat & available_errata

It also contains a few small fixes for issues found while
working on the new APIs.
Merge pull request #3729 from bbuckingham/cv-rework-available-content-2

Content Views Rework: APIs for available/current content