Use ApplicationSearch in Diffusion

Summary:
Ref T2625. Switches Diffusion to ApplicationSearch. Notes:

  - Rendering is a bit rough, I'll clean that up next.
  - Ordering is a bit arbitrary, also coming shortly.

Test Plan: Used `/diffusion/` to execute various searches.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2625

Differential Revision: https://secure.phabricator.com/D6917