Replace `Client` with `OrgUnit` and add basic setup.

Based on https://github.com/4teamwork/opengever.ogds.models/pull/18

- Add an upgrade step to migrate the database schema for the recent changes in `opengever.ogds.models`
- Replace `Client` with `OrgUnit` where it is used directly
- Add basic setup for `OrgUnit` and `AdminUnit` from *.json files in the generic setup profile.

Closes #501.