Mainly related to increasing compatibility with CE, and trying to avoid merge
conflicts.

1. Create an `EE::AuditorUser` module with auditor-specific methods. Mixed into
   the `User` model.

2. Create an `EE::User` module with EE-specific user methods. Mixed into the
   `User` model.

3. Don't block creation of regular users when the auditor addon is disabled (bug
   in original implementation).