R1:6d6211d44176

Use ApplicationTransactions in ApplicationEmail

Summary:
Ref T8498. I want to add Spaces to these, and the logic for getting Spaces right is a bit tricky, so swap these to ApplicationTransactions.

One new piece of tech: made it easier for Editors to raise DuplicateKeyException as a normal ValidationException, so callers don't have to handle this case specially.

One behavioral change: we no longer require these addresses to be at the `auth.email-domains` domains -- I think this wasn't quite right in the…