Organisational Unit and Administrative Unit 

This PR includes:
- Use 4teamwork/opengever.ogds.models#6 unit abstractions and service methods.
- Added upgrades step for the AdminUnit table migration (see 4teamwork/opengever.ogds.models#6)
- The `OrgUnitSelector` functionality (Viewlet, Helpers).
- Rework some tests and test setups according to the `OrgUnitSelector` integration,
- Replaced  `ContactInfo.list_assigned_users` with `assigned_users` methods on both units.
- Added SqlBuilders to create client, orgunits, adminunits etc.


@deiferni