R1:9b3d7b0dbaad

Make most Differential reads policy-aware

Summary: Ref T603. Makes the majority of reads policy aware (and pretty much all the important ones).

Test Plan:
- Created a comment with `differential.createcomment`.
- Created a new revision with `arc diff` in order to exercise `differential.creatediff`.
- Created an inline comment with `differential.createinline`.
- Added a comment to a revision.
- Edited an inline comment.
- Edited a revision.
- Wrote "Depends on ..." in a summary, saved…