Rework ogds and task functionality

Contains all improvements which are made during the task and OGDS rework, the last past weeks. 

- Add OGDSService which provides an entry point to query data.
- Add `OrgUnit` and `AdminUnit` model definitions, `OrgUnit` replaces `Client`.
- Move some functionality from `opengever.core` utility functions to methds on our model classes for better encapsulation.
- Add common base-class with query helper methods for all models.
- Add `ftw.builder` SQL-Builders for all model classes.
- Fixed backref name of the groups_user join table.