Engine: removing user model

The user model previously used by katello was moved to
a user_extensions module, which will be used to extend
the base User that is in the root namespace (::User).
Engine: content views: initial ui/model changes to support engine

This commit has the basic changes to support CRUD actions
in the UI for:
- content view definition
- content view
- filter
- filter rule

There are a few things that still need to be addressed:
- filter rules do not fully work due to issues with 'HashWithIndifferentAccess'
- need to fix a few 404 on loading assets (e.g. font, spinner)