Summary: Ref T3684 for discussion. This could be cleaned up a bit (it would be nice to draw entropy once per request, for instance, and maybe respect CSRF_TOKEN_LENGTH more closely) but should effectively mitigate BREACH.
Test Plan: Submitted forms; submitted forms after mucking with CSRF and observed CSRF error. Verified that source now has "B@..." tokens.
Reviewers: btrahan
Reviewed By: btrahan
CC: aran
Maniphest Tasks: T3684
Differential Revision: https://secure.phabricator.com/D6686
7298589c86