Application Emails - add datasource so we can have a typeahead

Summary: Ref T5039. This will be necessary for Herald integration so users can make rules like "if app email is one of x, y, or z add projects foo, bar, and metallica." I think its best to do an actual typeahead here -- users select full email addresses -- rather than support prefix, suffix, etc stuff on the email address. I think the latter approach would yield lots of confusion, as well as prevent us from (more) easily providing diagnostic tools about what happened when and why.

Test Plan: hacked a maniphest tokenizer to use this new datasource and it worked

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T5039

Differential Revision: https://secure.phabricator.com/D11546