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 method. This is presumably just a copy/paste issue from long ago.

Test Plan:

• Edited a revision in Differential.
• Dropped a file into the description of an Owners package.
  • Before change: this did not attach the file.
  • After change: the file now attaches properly and shows up as "Attached" in the file details.

Reviewers: chad, joshuaspence

Reviewed By: joshuaspence

Subscribers: joshuaspence

Maniphest Tasks: T9787

Differential Revision: https://secure.phabricator.com/D14493