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

Start with the linked example notebook and the IPython.display documentation, then review the related ipywidgets thread and nbconvert PR for existing approaches to static output. Done means the HiGlass dashboard is stored as PNG data and is visible when the notebook is previewed statically on GitHub.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.