Copy file attachment edges to new storage
Summary: Ref T13603. Copy all existing file attachment data into the new storage table.
Test Plan: Ran migration, checked attachment table for general reasonableness.
Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam
Maniphest Tasks: T13603
Differential Revision: https://secure.phabricator.com/D21818