Added SHA256 hashes for original and patched files in FileDiffs.

Review Request #1832 — Created Jan. 23, 2020 and updated — Latest diff uploaded

Review Board
Back in Review Board 2.0, we introduced SHA1 hashes in FileDiffs, which
were in part used to determine if filediffs and interfilediffs matched.
SHA1 is not technically safe anymore (though there's little harm it can
do in our situation, aside from generate patch errors in the diff
viewer), and we want to move to SHA256 where possible.

This change adds storage and properties for SHA256 hashes on FileDiff,
and updates our FileDiff comparison logic to prioritize them over SHA1
hashes (though it will still check SHA1 if the SHA256 hashes are
missing, since they're created only when viewing the diff for the first

The comparison logic has been moved into a new `get_filediffs_match()`
function, and unit tests were added.

Testing Done:
Unit tests pass.

Didn't see any regressions when viewing interdiffs.

Verified that FileDiffs were recording the SHA256 hashes.

Reviewed at