Prevent application email addresses from shadowing user email addresses

Summary:
Fixes T13234. Don't let application email addresses be configured with user addresses. This might prevent an unlikely bit of mischief where someone does this intentionally, detailed in T13234.

(Possibly, these tables should just be merged some day, similar to how the "Password" table is now a shared resource that's modular enough for multiple applications to use it.)

Test Plan: {F6132259}

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13234

Differential Revision: https://secure.phabricator.com/D19974