Move Slowvote rendering into SearchEngine

Summary:
Ref T4986. This adds a bit of structure for handles, since we used to have Controller utilities but no longer do.

Hopefully these will start going faster soon...

Test Plan:

• Checked feed for collateral damage.
• Checked slowvote for collateral damage.
• Made a slowvote panel.

{F151550}

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4986

Differential Revision: https://secure.phabricator.com/D9010