Implement ApplicationSearch in People

Summary:
Ref T2625. Fixes T2812. Implement ApplicationSearch in People.

{F44788}

Test Plan: Made People queries. Used Conduit. Used `@mentions`.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T2625, T2812

Differential Revision: https://secure.phabricator.com/D6092