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