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