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