Modernize DifferentialRevisionSearchEngine

Summary:
Ref T10939. Ref T4144. This moves the revision SearchEngine to modern code so I can add some kind of bucketing layer on top of it.

This seems to have worked pretty cleanly. One thing is that I removed the ability to search for "pending drafts":

• This was added in D1927 from a bootcamp task, was an indirect solution to a questionable problem, and almost certainly would not meet the bar today.
• Later, in D3324, we added the icons to the list. I think this is a better solution in general. In particular, it specifically addressed the query being kind of junky.
• At the time, Differential had a prebuilt "Drafts" filter. This was removed in D6347 with the move to ApplicationSearch, which simplified the large number of prebuilt filters. Although we got a lot of feedback about that, none requested that the drafts filter be restored.

Test Plan: Searched for responsible users, subscribers, orders, projects, repositories.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4144, T10939

Differential Revision: https://secure.phabricator.com/D15921