Proof of concept mitigation of BREACH

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