Make e-mail address on Person unique

The e-mail address field should be unique, also for perosns, and we should prohibit the use of e-mail addresses for multiple persons.

Re-using e-mail addresses is a security issue because it simplifies account hijacking scenarios.

Alternatively, we could ensure that for all account-related features, e.g. OpenID claims, the mai laddress from the linked use is used, and only enforce uniqueness for user mail addresses.