Provide an attached-to-visible-object policy exception for files

Summary:
Ref T603. This uses the existing edges (from Conpherence) to record that a file is attached to an object, and uses those edges to create a policy exception: if you can view an attached object, you can view a file.

I'm going to combine this with restrictive defaults to satisfy the other half of the equation (that files you attach to a conpherence usually shouldn't be public by default).

Test Plan:

• Loaded /files/.
• Uploaded a file to a Conpherence, looked at it in Files, saw the attachment.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7182