R1:e45ffda55a9a

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…