Move most remaining sha1() calls to HMAC

Summary:
  - For context, see T547. This is the last (maybe?) in a series of diffs that
moves us off raw sha1() calls in order to make it easier to audit the codebase
for correct use of hash functions.
  - This breaks CSRF tokens. Any open forms will generate an error when
submitted, so maybe upgrade off-peak.
  - We now generate HMAC mail keys but accept MAC or HMAC. In a few months, we
can remove the MAC version.
  - The only remaining callsite is Conduit. We can't use HMAC since Arcanist
would need to know the key. {T550} provides a better solution to this, anyway.

Test Plan:
  - Verified CSRF tokens generate properly.
  - Manually changed CSRF to an incorrect value and got an error.
  - Verified mail generates with a new mail hash.
  - Verified Phabricator accepts both old and new mail hashes.
  - Verified Phabricator rejects bad mail hashes.
  - Checked user log, things look OK.

Reviewers: btrahan, jungejason, benmathews

Reviewed By: btrahan

CC: aran, epriestley, btrahan

Maniphest Tasks: T547

Differential Revision: 1237

src/docs/userguide/diffusion.diviner

@@ -58,7 +58,7 @@ of 7-character hashes:
 
 Because 7-character hashes are likely to collide for even moderately large
 repositories, Diffusion generally uses either a 16-character prefix (which makes
-collisions very unlikely) or the full 40-character SHA1 (which makes collisions
+collisions very unlikely) or the full 40-character hash (which makes collisions
 astronomically unlikely).
 
 = Adding Repositories =