Feature: Fast manifest import 

This pull requests breaks up the manifest import process into two different steps.

Previously when you imported a manifest the following would happen:
- Import into candlepin
- Create katello products based off those in candlepin
- Scan the cdn to verify Release and arch combinations
- Create a repository for each release and arch combination
- User would enable desired repositories

New Process:
- Import into Candlepin
- Do nothing :)
- User can go to 'red hat repositories page' and enable a specific content set (called repository set) under a product
  - This scans the cdn for that specific content's arch and release combinations
  - Repositories are created for each release and arch combination
- User would enable desired repositories within that content set 

The result is instead of taking  30-90 minutes to import a manifest, it now takes roughly 10 seconds.  Enabling a specific content set takes 10-20 seconds.

Content sets can also be disabled if all the repositories beneath them are all disabled.  This completely deletes the repositories from katello & pulp