Engine: Authorization: use ::User vs User

Updating references to User to explicitly point to the core
user (::User)
Engine: fixes #3332: Roles initial conversion

This is the initial conversion of the katello roles ui to work
within the engine.  This includes CRUD actions for roles
and permissions.
Merge pull request #3258 from bbuckingham/engine-roles

Engine: fixes #3332: Roles initial conversion