ContactInfo utility refactoring

This PR contains some small refactorings on the `ContactInformation` utility. Centralized:
* Dropped unnecessary `get_email`
* Consistent use of `filter_by` for single table queries
* Manual revert of 26302a5. That "workaround" is not required.
* Refactor `get_client_of_inbox` and `get_group_of_inbox`
* Use `is_one_client_setup` where appropriate.
* Use `count` instead of fetching the record when only existence check is needed

All changes are only refactorings, the functionality of the contact_information utility is not affected.

@lukasgraf