Removes the old auth models and their use.

Removes all of the old auth models, and contains a migration to delete the now unused tables.  There are a few components still depending (secretly behind the scenes) on PackageRole and as a result for this PR to be complete it needs to re-implement `number_administered_packages`. Also breaks ```stats```.

Should allow for close of #1903 as there's another PR for renaming new_authz.