Add an nbgitpuller generator to the Hub

[choldgraf]

We should make it easier for people to generate nbgitpuller links for a JupyterHub. I opened up jupyterhub/nbgitpuller#154 to discuss in nbgitpuller, but we could think of other solutions here as well.

A few thoughts:

• A Lab / Notebook extension that lets you quickly generate an nbgitpuller link for the current hub / content file
• A control panel tab that links externally to nbgitpuller.link with the hub info pre-filled out
• A full UI for the control panel tab per Create an nbgitpuller tab in the JupyterHub admin panel jupyterhub/nbgitpuller#154