Add HTTP basic auth to language servers

Created by: chrismwendt

All customers that I know of only deploy language servers inside a VPN, but it would be useful for the language servers to support HTTP basic auth for both the case when they're accessible from the Internet and for additional security.

Tech sketch:

• Add docs for requiring HTTP basic auth to language servers through nginx https://github.com/sourcegraph/sourcegraph-go/pull/37 (TODO move to sourcegraph/sourcegraph)
• Update extension READMEs to mention how to add HTTP auth:
  • Create an .htpasswd file with a password
  • Configure nginx to require basic auth before routing traffic to the language server
  • Add the basic auth password to the {go,typescript,python}.serverUrl setting

cc @KattMingMing @slimsag