Require CSRF submission to verify email addresses

Summary: If an attacker somehow intercepts a verification URL for an email address, they can hypothetically CSRF the account owner into verifying it. What you'd do before (how do you get the link?) and after (why do you care that you tricked them into verifying) performing this attack is unclear, but in theory we should require a CSRF submission here; add one.

Test Plan: {F118691}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D8351