Make sure email addresses on users/contacts are unique

https://trello.com/c/355SdNHp/52-2h-email-adresses-should-be-unique

On users we already had a UNIQUE index on (email, active), I didn't add one on contacts since we don't have an active flag there and probably want email addresses to be reusable after a contact is soft-deleted. But both models now validate the uniqueness on the model level (so ignoring the deleted records).