R1:3a727c31e2f9

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…