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.