Add generic "attributes" storage to inline comment tables

Summary: Ref T13513. This plans for "currently editing", character range comments, code suggestions, document engine tracking. And absolutely nothing else.

Test Plan:

• Ran bin/storage upgrade -f, got a clean upgrade.
• Created and submitted some inline comments; nothing exploded.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13513

Differential Revision: https://secure.phabricator.com/D21184