Add database storage for a dedicated file attachment table

Summary: Ref T13603. Prepare to move this relationship out of edge storage into dedicated storage so it is easier to formalize better in the UI.

Test Plan: Ran bin/storage upgrade.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13603

Differential Revision: https://secure.phabricator.com/D21813