Deactivate users

Closes #89 (closed). Deactivating a user should behave like deleting the user, but without actually deleting any data.

This introduces user states, for now just ACTIVE and DEACTIVATED. In the near future I also plan to add a DELETED state to soft-delete users while preventing reuse of their UID and loginname. Later I see states like INACTIVE (like DEACTIVATED but entered automatically and reset upon login) und DELETION_IN_PROGRESS (when waiting for all services to delete data of a user) to implement automatic account cleanup after a long time of inactivity.