mark externalURL as required
Created by: nicksnyder
Adds a doc comment and marks externalURL as required.
This is one small step to help https://github.com/sourcegraph/sourcegraph/issues/3899. It does not prevent the form from being submitted though. This isn't perfect because there is a styling issue that causes the tooltip to get clipped (and I am not going to try to fix that in this PR).
Still, I think this is better than nothing.
Test plan: tested locally