[FEATURE] Introduce the path_segment field that allows to give proper URLs yourself, Also a lot of cleanup was done

This commit adds a new field and a new record type, the department.
It comes as a replacement for the area text field and allows for
department records to be created. This also introduces a migration
wizard, that can create new departments out of the old area entries.

To prevent users from clicking the "Create New" button on the index
page of the backend module, we make this button admin only