Hide "Configure external services to connect to Sourcegraph." after "other" external service is added

Created by: nicksnyder

fixes https://github.com/sourcegraph/sourcegraph/issues/1839

Also improves the banner message.

Previously Phabricator was not included in this this, and I don't know if that was intentional or not, but it seems wrong so I also changed that. I believe that we have a customer who doesn't use it as a code host, but in general it may be used as a code host.