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 don't have SHA256 for some reason. Got two valid files with two copies of the same data, as expected.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12464

Differential Revision: https://secure.phabricator.com/D17620