auth: Map Keycloak user permissions to Django ones
This was the last task remaining to implement in order to close #2048 (closed).
Keycloak user permissions are named Roles in Keycloak semantics.
Extract them from each decoded access token and override methods from
django.contrib.auth.models.PermissionsMixin
in order to manipulate them
as Django user permissions.
Closes #2247 (closed)
Migrated from D3304 (view on Phabricator)