R1:b256f2d7b2db

Prepare UserPreferences for transactions

Summary:
Ref T4103. This give preferences a PHID, policy/transaction interfaces, a transaction table, and a Query class.

This doesn't actually change how they're edited, yet.

Test Plan:
- Ran migrations.
- Inspected database for date created, date modified, PHIDs.
- Changed some of my preferences.
- Deleted a user's preferences, verified they reset properly.
- Set some preferences as a new user, got a new row.
- Destroyed a user, verified their preferences were…