Add Ferret engine index support to Differential

Summary: Ref T12819. Adds storage and indexing for the Ferret engine to Differential.

Test Plan: Ran bin/search index D123 --force, saw indexes appear in database. No UI/user impact yet.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12819

Differential Revision: https://secure.phabricator.com/D18540