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 destroyed.
• Sent Conpherence messages.
• Send mail.
• Tried to edit another user's settings.
• Tried to edit a bot's settings as a non-admin.
• Edited a bot's settings as an admin (technically, none of the editable settings are actually stored in the settings table, currently).

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4103

Differential Revision: https://secure.phabricator.com/D15991