Remove incorrect config validation and fix external services migration
Created by: nicksnyder
Two commits:
- Fixes external services migration, which I tested before integrating Stephen's config changes and didn't work after those changes.
- Fixes https://github.com/sourcegraph/sourcegraph/issues/1408 by removing validation that was incorrect. The logic in ContributeValidator should only validate the cfg itself, not state in the DB (what it was doing before). The external service state in the DB is validated when the external service is created or edited.