Add default policy to Files application

Summary: **WIP** This adds default capability to the Files application

Test Plan: Set default to public, go to Files page, see public preset. Upload File. Doesn't work.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6564

Differential Revision: https://secure.phabricator.com/D10888