Allow bin/config to affect database configuration and migrate between local and database configuration

Summary: Fixes T4018. Basically hits the bullet points in that task description except the "ideally" one.

Test Plan:
ran bin/config migrate and saw sensible output.

~> ./bin/config migrate
Migrating file-based config to more modern config...
Skipping config of source type PhabricatorConfigDatabaseSource...
Skipping config of source type PhabricatorConfigLocalSource...
Skipping config of source type PhabricatorConfigDefaultSource...
Done. Migrated 0 keys.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: hach-que, epriestley, Korvin

Maniphest Tasks: T4018

Differential Revision: https://secure.phabricator.com/D10490