access: new has_(super)_admin_role methods

* PEP8/257 improvements.
* Adds simple Flask-Admin interface for access models.
* Cleans legacy `webuser` usage.
> As part of #2340 we could factor it out to store it in the model linked with user table?

Yes, that would be nice. If you see that you need to filter by some value in user settings, than it should be probably factored out.