Make file policies for emailed files more consistent

Summary:
Fixes T7712. Currently, files sent via email get default policies, like they were dragged and dropped onto the home page.

User expectation is better aligned with giving files more restrictive policies, like they were draggged and dropped directly onto an object.

Make files sent via email have restricted default visibility. Once we identify the sender, set them as the file author. Later, the file will become visible to other users via attachment to a task, revision, etc.

Test Plan: Sent some files via email; verified they got restrictive policies, correct authorship, and appropriate object attachment.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7712

Differential Revision: https://secure.phabricator.com/D12255