Execute nbgitpuller via menu

### Proposed change

the nbgitpuller tool is powerful but not very convenient for end-users. We would like to offer to all users of our jupyterhub instance the ability to clone notebooks & data via `nbgitpuller` using a simple button click.

When a set of notebooks (etc.) is known upfront, it would be very useful to be able to use "pre-canned" nbgitpuller links through jh's config infrastructure, and populate a dedicated menu with it. Users then simply click an individual menu item from that menu to clone the corresponding data into their respective home directory.

### Alternative options

The alternative is to let users generate the links themselves and paste them into their browser bar, but that's hard to remember and less convenient compared to using the menu.

### Who would use this feature?

Such a GUI would be useful whenever a jh instance wants to offer common sets of notebooks to all its users.