Assign PHIDs to all diffs

Summary:
Ref T1049. Ref T2222. DifferentialDiff does not currently have a PHID, but we need it for Harbormaster and ApplicationTransactions. See some discussion in D7501.

(I split the SQL into two sections so we can't fail in the middle. At some point, I'd like to do a pass on the migration stuff and get this happening automatically, and also simplify the PatchList.)

Test Plan:

• Ran bin/storage upgrade.
• Checked for valid PHIDs in the database.
• Used phid.query to look up a diff by PHID.
• Created a new diff and verified it got a PHID.

Reviewers: btrahan, hach-que

Reviewed By: btrahan

CC: aran, vrana

Maniphest Tasks: T2222, T1049

Differential Revision: https://secure.phabricator.com/D7513