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