Support CSRF for logged-out users
Summary: Fixes T4339. If you're anonymous, we use a digest of your session key to generate a CSRF token. Otherwise, everything works normally.
Test Plan: Logged out, logged in, tweaked CSRF in forms -- I'll add some inlines.
Reviewers: btrahan
Reviewed By: btrahan
CC: aran
Maniphest Tasks: T4339
Differential Revision: https://secure.phabricator.com/D8046