Feature Request: Extend project user API for batch processing

[ChristopherFoerster]

As CCM does not support LDAP groups we would like to ask for support of batch processing (multiple users) for the APIs:
POST: /api/v1/projects/{project_slug}/users
DELETE: /api/v1/projects/{project_slug}/users

That would reduce our high load of REST requests by 95% as today we remove and add all users project by project at every sync. Otherwise we would need quite complex logic to add users, remove users, update users based with many compares and corner cases like duplicated users in different access groups etc.