R1:58011a4e8e7f

Upgrade File content hashing to SHA256

Summary:
Ref T12464. This defuses any possible SHA1-collision attacks by using SHA256, for which there is no known collision.

(SHA256 hashes are larger -- 256 bits -- so expand the storage column to 64 bytes to hold them.)

Test Plan:
- Uploaded the same file twice, saw the two files generate the same SHA256 content hash and use the same underlying data.
- Tried with a fake hash algorihtm ("quackxyz") to make sure the failure mode worked/degraded correctly if we…