R1:13dae0519368

Make most file reads policy-aware

Summary: Ref T603. Swaps out most `PhabricatorFile` loads for `PhabricatorFileQuery`.

Test Plan:
- Viewed Differential changesets.
- Used `file.info`.
- Used `file.download`.
- Viewed a file.
- Deleted a file.
- Used `/Fnnnn` to access a file.
- Uploaded an image, verified a thumbnail generated.
- Created and edited a macro.
- Added a meme.
- Did old-school attach-a-file-to-a-task.
- Viewed a paste.
- Viewed a mock.
- Embedded a mock.