Provide better support for air-gapped instances

Created by: ryankscott

By default Sourcegraph instances require access to sourcegraph.com in order to fetch extensions from our extension registry. In an air-gapped instance, this isn’t possible and so we recommend that customers use their own private extension registry. However, by default this registry has no extensions and core features of Sourcegraph don’t function (e.g. code intelligence).

For more see relevant RFC