R1:2a0af8e2995d

Add email invites to Phabricator (logic only)

Summary:
Ref T7152. This builds the core of email invites and implements all the hard logic for them, covering it with a pile of tests.

There's no UI to create these yet, so users can't actually get invites (and administrators can't send them).

This stuff is a complicated mess because there are so many interactions between accounts, email addresses, email verification, email primary-ness, and user verification. However, I think I got it right and got test…