use `OGDService` to query user and client data.

 * Use the branch `pg_user_model_extraction` from the package `opengever.ogds.models`
refactor, ogds_service#all_users to load all users.

This method is no longer cached. The usages of `all_users` were mostly
vocabularies, which were cached themselves.

- refactor, UserDict is no longer necessary.
revert `get_group_of_inbox` refactoring to return a primitive String

This is necessary for caching purposes. @ram.cache won't allow SQLAlchemy objects.
refactor, ogds_service#inactive_users to load all inactive users.

Additionaly we use label instead of the `contact_info.describe` method.
Added OrgUnitSelector functionality

The OrgUnitSelector should replace ...
OrgUnitSelector tests: fixed typo and add ordering tests.
Adjust the test setup and helpers that all the tests can handle with the OrgUnitSelector.

Some tests had need some ajustments in the setup as well.
Fixed markup in ou_selector_viewlet.
Temporarily disable some tests, which has to be reworked and reviewed
after implementing the OrgUnitSelector stuff.
Dropped senseless tests.

When testing this feature the test should be written new completly.
Use `assigned_users` on a OrgUnit instead of `IContactInformation.list_assigned_users`.
Added `current_admin_unit` getter.

Including corresponding registry entry (Upgradestep is missing).
Add db-migration, create admin_unit table and foreign key.
Add upgrade step to create IAdminUnitConfiguration registry entry.
TMP: Dropped no longer used contact info method `list_assigned_users`.
Add sql-builders.

Move existing builders to builders package.
Display only current admin unit's users on the sharing view.

Old behavior was to use the client's users.
Replace ContactInformation is_one_client_setup with ogds service.
Merge pull request #268 from 4teamwork/pg_organisational_unit_selector

Organisational Unit and Administrative Unit