Drop `get_client_id` and `IBaseClientID`

Replaces #478. Based on https://github.com/4teamwork/opengever.ogds.models/pull/15

- Remove `get_client_id` 
- remove `IBaseClientID` registry entries

Furthermore tests are refactored, `FunctionalTestCase` sets up a minimal fixture consisting of an org-unit an admin-unit and an ogds-user.

Closes #434.