R1:5ca419174a6c

Proide application-based handle loads; load Slowvote handles from the application

Summary:
Ref T2715. This is pretty straightforward, I think. Notes:

- Long term, I want to replace `PhabricatorObjectHandleData` with `PhabricatorObjectQuery` and `PhabricatorHandleQuery`. The former's name is a relic of old Facebook stuff and unusual now that everything else uses normal queries.
- I simplified the amount of work applications need to do in order to populate handles. The should just need to set names and…