Disable extension registry unless enableLegacyExtension is true or we're on dot-com

Created by: philipp-spiess

We need to keep the registry running for sourcegraph.com since all <4.0 instances will depend on it. For other instances we do not want to expose it (=the private extension registry) unless enableLegacyExtension is true.

/cc @muratsu @jjinnii @ryankscott