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 general case. It's OK to require users to have @mycompany.com addresses but add @phabricator.mycompany-infrastructure.com addresses here if you want.

Test Plan:

• Tried to create a duplicate email.
• Tried to create an empty email.
• Tried to create an invalid email.
• Created a new email.
• Deleted an email.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8498

Differential Revision: https://secure.phabricator.com/D13246