R1:fe0ca0abf267

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…