R1:5aae89babb73

Fix file PHID extraction in Owners and Differential

Summary:
Ref T9787. To fix this, I want to change how file PHIDs are extracted slightly: specifically, I'm going to extract them later in the editing process.

Before doing this, clean up a couple of bad implementations:

- Owners extracts its description as a file PHID. This is an error.
- Extract the description as a remarkup block instead.
- Add an edge table so stuff like file attachment works properly.
- Differential has a no-op extract…