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 URIs in most cases.

Test Plan: Used phid.lookup and phid.query to load slowvote handles. Browsed around to load other handles.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2715

Differential Revision: https://secure.phabricator.com/D6508