Implement ApplicationSearch in Files

Summary: Ref T2625. Ref T1163. A couple of small generalization nudges, but this is almost entirely straightforward.

Test Plan: Executed various File queries.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1163, T2625

Differential Revision: https://secure.phabricator.com/D6091