Fix login validations

The validations of user logins, identities and email addresses were somewhat interlinked but not in a clean way and broken.

Now we have the following responsibilities:

User:
 * validates that the login format is valid
 * validates that the identity (default email address) for the user is valid

Identity:
 * validates that the email address is available
 * validates that the source and destination emails are valid

Email Addresses:
 * validate the format of an email address
 * for source email addresses (LocalEmail) they also make sure they end in the providers domain.

Errors are handed on from Email Addresses to Identities to Users when needed.