Support new syncer in sourcegraph.com deployment

Created by: tsenart

Tasks:

• Endpoint for lazily inserting new repositories.
• Keep CloneURL up to date in database.
• Stretch Lazily refresh data

In #2025 (closed), we deferred working on making sourcegraph.com work with the new syncer. This needs to be done still.

Related to this, since we removed support for the old OtherExternalServicesSyncer, #3457 (closed) depends entirely on this.