roles: make use of Ring::Role enum in models

::roleData() should now return appropriate data when using the
Ring::Role enum where applicable.

In order to not break current behaviour, all model specific roles
have been kept; however their values have been prepended with
Ring:: Role::UserRole in order to guarantee that there are no
collisions with the Ring::Role enum. Explicit enum values are
also dropped since the values are not the same anyways.

All clients should test that no behaviour is broken.

Issue: #81198
Change-Id: Id8ca13ab47be355a710de7fc3dd2d048ec04db30