R1:c013108675ef

Modernize all Herald DifferentialDiff fields

Summary: Ref T8726. This deals with all the Differential diff fields, same deal as previous changes.

Test Plan:
- Wrote a rule with every field.
- Migrated it.
- Saw the same rule working.
- Rigged the hell out of transcripts (diffs normally do not generate transcripts, because the only action is "block" and they don't exist yet when Herald runs).
- Verified that all fields looked sensible in the transcript.

Reviewers: btrahan

Reviewed By: btrahan…