Allow users to set view policies on files explicitly

Summary: Ref T4589. Allow users to adjust visibility settings on files explicitly. This makes it easier to understand and manage upcoming changes in T4589.

Test Plan: Changed the view policy for a file several times.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4589

Differential Revision: https://secure.phabricator.com/D10129