Give files uploaded to objects a very restrictive view policy

Summary:
Fixes T4589. This implements much better policy behavior for files that aligns with user expectations.

Currently, all files have permissive visibility.

The new behavior is:

• Files uploaded via drag-and-drop to the home page or file upload page get permissive visibility, for ease of quickly sharing things like screenshots.
• Files uploaded via the manual file upload control get permissive visibility by default, but the user can select the policy they want at upload time in an explicit/obvious way.
• Files uploaded via drag-and-drop anywhere else (e.g., comments or Pholio) get restricted visibility (only the uploader).
  • When the user applies a transaction to the object which uses the file, we attach the file to the object and punch a hole through the policies: if you can see the object, you can see the file.
  • This rule requires things to use ApplicationTransactions, which is why this took so long to fix.
  • The "attach stuff to the object" code has been in place for a long time and works correctly.

I'll land D8498 after this lands, too.

Test Plan:

• Uploaded via global homepage upload and file drag-and-drop upload, saw permissive visibility.
• Uploaded via comment area, saw restricted visibility.
• After commenting, verified links were established and the file became visible to users who could see the attached object.
• Verified Pholio (which is a bit of a special case) correctly attaches images.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4589

Differential Revision: https://secure.phabricator.com/D10131