Fix externalURLs alert logic

Created by: felixfbecker

Fixes https://github.com/sourcegraph/customer/issues/118

The issue was that the code assumed the array always had an element, but it doesn't need to have. I also noticed that we didn't check whether we actually support the code host. With this PR, we take the first code host from external links that we support, and show nothing if there is none.

cc @dadlerj