When a user changes to a verified primary address, mark their account as verified

Summary:
Ref T12635. See that task for discussion.

You can currently end up with a verified primary address but no "verified" flag on your account through an unusual sequence of address mutations.

Test Plan:

• Registered without verifying, using address "A".
• Added a second email address, address "B".
• Verified B (most easily with bin/auth verify).
• Changed my primary email to B.
• Before patch: account not verified.
• After patch: account verified.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12635

Differential Revision: https://secure.phabricator.com/D17785