R1:1752be630c28

Improve handle semantics with HandlePool / HandleList

Summary:
Ref T7689, which discusses some of the motivation here. Briefly, these methods are awkward:

- Controller->loadHandles()
- Controller->loadViewerHandles()
- Controller->renderHandlesForPHIDs()

This moves us toward better semantics, less awkwardness, and a more reasonable attack on T7688 which won't double-fetch a bunch of data.

Test Plan:
- Added unit tests.
- Converted one controller to the new stuff.
- Viewed countdown lists…