add interactive 2D scatterplot option to web-app

[moustakas]

@tanveerkarim had a nice suggestion for the web-app: At the top-level page, allow users to generate different kinds of scatterplots based on the myriad quantities in the database, the way Topcat and other tools allow you to do. An example of this capability we looked at together was this exoplanets tool, although many others exist--
https://exoplanet.eu/plots/

I'm not sure what tool will work the best, but https://plotly.com/python or https://bokeh.org/ may be possible options.