Replace Remarkup calls to PhabricatorHash::digest() with SHA256

Summary:
Ref T12509. Many of the calls to HMAC+SHA1 are just to compute cachekeys for remarkup objects.

Make these use HMAC+SHA256 instead. There is no downside to swapping these since they just cause a cache miss in the worst case.

I also plan to get rid of PhabricatorMarkupInterface eventually, but this doesn't go that far.

Test Plan: Browsed some different types of documents (tasks, legalpad documents, phame blogs / posts, pholio mocks, etc).

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12509

Differential Revision: https://secure.phabricator.com/D17631