Create an nbgitpuller tab in the JupyterHub admin panel

In the [tljh-repo2docker repository](https://github.com/plasmabio/tljh-repo2docker) we saw a nice pattern where they let users build a docker image for the JupyterHub via an admin panel button that opens up a small form.

I think that we should have a similar button for `nbgitpuller` links so that administrators can quickly fill out the form *from within their JupyterHub* and get a link in return.

A simple way to do this might even be to just IFrame the `nbgitpuller.link` page, but with a few fields pre-filled (such as the current jupyterhub).

What do folks think?

Environments tab:


would open up a form like:


(but for the purposes of building an interact link instead of building an image)