R1:5677cd23bdec

Add storage and classes for CustomField in Differential

Summary: Ref T3886. Adds the storage, indexes, and storage classes for modernizing Differential custom fields.

Test Plan: Ran `storage upgrade`.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3886

Differential Revision: https://secure.phabricator.com/D7138