Add `viewPolicy` and `attachedToObjectPHID` to PhabricatorFile

Summary:
Ref T603. Principally, I want to implement the rule "when you upload a file to an object, users must be able to see the object in order to see the file", since I think this is strongly in line with user expectation. For example, if you attach a file to a Conpherence, it should only be visible to members of that thread.

This adds storage for policies, but doesn't do anything interesting with it yet.

Test Plan: Ran `bin/storage upgrade`.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7175