fixes #7705 - adding errata counts to content host details

This brings errata, repository association to errata, and applicable errata for systems into
the database. Also remove the Errata class that was based on Elasticsearch.

In addition:

* Binds consumer repositories in pulp based on the library instance.
  * Looks up current applicable errata via its current bound repositories
  * This allows future stories to look up errata for different environments
* removes errata from content search
* bulk actions when 'select all' mode was enable was broken, fixed that
* fixed details nutupane search using the main table's search box (such as on the bulk errata search)

The goal of all of this was to not change the v2 api at all (other than the added attribute that issue 7705 requested), just change the underlying implementation. 

TODO:

- [x] fix bulk errata actions
- [x] fix existing tests
- [x] write lots of tests
- [x] import new applicability after sync
- [x] write upgrade rake task
Do the unit handlers follow this pattern? Is it possible to avoid referencing `Katello.pulp_server.extensions` by defining a unit_type and letting the included Glue module set this up?