use nbgitpuller in for binderhub links

[scottyhq]

if we can use nbgitpuller in the binderhub launch buttons then it will be easier to ensure people have same environment whether content is launched via binderhub or jupyterhub.

I naively tried this

jupyterbook-template/book/_config.yml

Line 28 in b8c6246

#binderhub_url: https://aws-uswest2-binder.pangeo.io/v2/gh/pangeo-data/notebook-binder/2021.02.02?git-pull?repo=https://github.com/uwhackweek/jupyterbook-template

but think we'll need some upstream changes jupyter-book/jupyter-book#1235

[jomey]

We have removed the use of BinderHub with #150