Allow users to set an explicit visibility for manual file uploads at creation time

Summary: Ref T4589. Depends on D10129. In addition to letting users change the visibility policy for files, also allow them to choose a policy explicitly when a file is uploaded.

Test Plan: Uploaded several files using the plain old uploader, saw appropriate visibility policies applied.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4589

Differential Revision: https://secure.phabricator.com/D10130