Let feed panels render something meaningful-ish

Summary:
Ref T4986. We need to introduce alternate views to make this more pleasant, but let rendering move to engines so it can be shared between panels and controllers.

I also moved some of the pagination logic in to avoid duplicating that.

So far, only Feed works. I'm going to do these gradually since we have ~40-50 of them.

Test Plan:

• Used global search to check for collateral damage.
• Used not-global search too.
• Used normal feed.

{F151541}

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: chad, epriestley

Maniphest Tasks: T4986

Differential Revision: https://secure.phabricator.com/D9008