access: fix PEP8 and PEP257 for models

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
access: new has_(super)_admin_role methods

* Adds new properties to `User` model in order to ease access control
  calculations.  (closes #2509)

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
access: Flask-Admin interface addition

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
legacy: webuser usage cleanup

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>