R1:42af0d66d9ec

Use ApplicationSearch in Feed

Summary: Ref T2625. This doesn't do anything fancy, but gives feed a little more flexibility.

Test Plan: Viewed `/feed/`.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T2625

Differential Revision: https://secure.phabricator.com/D6681