Expose Differential actions for "transaction.search" in a basic way

Summary:
See PHI725. Ref T13151. These actions are somewhat unusual and I considered different ways to represent them (make them look like "status" transactions; build multiple synthetic transactions) but ultimately landed on the simplest approach of just exposing them more or less as they exist internally.

I haven't included data for any of them. Most don't really have any data, but "accept" does. I'm holding off on providing more data until after T731, which may shake up the internal format.

Test Plan: Applied most of these transactions against a revision, queried for it with transaction.search, got distinguishable transactions out.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13151

Differential Revision: https://secure.phabricator.com/D19509