R1:dadd9a9dd98d

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.