Need to accept HEAD requests to validate Sourcegraph domain on VSCode Marketplace

Created by: muratsu

We currently don't allow HTTP HEAD calls on Sourcegraph.com.

eg the following returns 405 (Method Not Allowed)

curl -I https://sourcegraph.com/

According to this thread, we need to return 200 OK, in order to be able to validate our domain name on the VS Marketplace. This is an important step because it lets us affirm our identity to become a verified publisher.