Prep user table for default images

Summary: Ref T10319. Adds in database columns for upcoming default generated avatar support.

Test Plan: Ran storage upgrade, log into local site to verify it didn't blow up.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T10319

Differential Revision: https://secure.phabricator.com/D17459