Display PNG preview when notebook is shown statically

This notebook, for example, has a few HiGlass views but they're not visible when previewed in GitHub: https://github.com/reservoirgenomics/paper-data/blob/master/boyle-et-al-2019/Boyle%20et%20al.%202019.ipynb

We should store a PNG of the higlass dashboard and preview that.

Relevant thread: https://github.com/jupyter-widgets/ipywidgets/issues/2280

A relevant PR: https://github.com/jupyter/nbconvert/pull/901

Docs on the IPython.display object that needs to be updated: once we have image data: https://ipython.readthedocs.io/en/stable/api/generated/IPython.display.html#IPython.display.display