R1:d017f3f21021

Double-write file attachment to old "edge" storage and new "attachment" storage

Summary: Ref T13603. This adds a second write to new "attachment" storage to all writers except one in Paste, which creates the file inline.

Test Plan:
- Updated a macro image, confirmed a write to "attachment" storage (transaction pathway).
- Updated a blog profile image, confirmed a write to "attachment" storage (legacy pathway).

Maniphest Tasks: T13603

Differential Revision: https://secure.phabricator.com/D21816