Add external link to nav bar dropdown

[JB515]

I've added a couple of external links to startups/open source projects my nav bar using the method described in #579 which is working well. I now need to add some more and as they're all to 'end user' stuff, I'd like to group them into a dropdown. I can make a dropdown for pages on my site, but this isn't working with external links. Is there a way to do this?

[george-gca]

I believe this can be easily done by creating a copy of _pages/dropdown.md and inserting the links there. Just set the correct permalink attributes.