Add ability to launch each vignette notebook on MyBinder.org

[chmreid]

binder (mybinder.org) is a site that can turn a repository full of notebook files into an interactive collection of notebooks. (The underlying technology it runs on is binderhub.)

We should add an "Open in Binder" badges to each of our vignette notebooks to allow users to launch the notebook in the cloud and run the notebook/associated analysis using HCA data in the cloud.

[chmreid]

Potential hangup with this: mybinder.org expects a monolithic requirements.txt covering all notebooks in the entire repo. We have notebooks organized into folders with their own requirements.txt. Aaarrrrgh.