R1:93e6dc1c1d69

Upgrade object reply addresses to SHA256 and remove "phabricator.mail-key"

Summary:
Ref T12509.

- Upgrade an old SHA1 to SHA256.
- Replace an old manually configurable HMAC key with an automatically generated one.

This is generally both simpler (less configuration) and more secure (you now get a unique value automatically).

This causes a one-time compatibility break that invalidates old "Reply-To" addresses. I'll note this in the changelog.

If you leaked a bunch of addresses, you could force a…