Add a mybinder.org tab to the link builder page

[choldgraf]

I think that the interactive link builder page is a super helpful way to build a link: https://jupyterhub.github.io/nbgitpuller/link

In this discourse post we discussed how people could de-couple their environment repositories from their content repositories using nbgitpuller. It was suggested that we could upgrade the nbgitpuller form with a "mybinder.org" tab. We should do that!

Maybe this is something @GeorgianaElena would be interested to work on? It shouldn't be too much work to extend the current form to add another tab

[GeorgianaElena]

Happy to help 🌞

[yuvipanda]

This is awesome, @GeorgianaElena I'd be very happy to review + merge :)