Create "preview images"

[stscijgbot-jwql]

Issue JWQL-259 was created on JIRA by Bryan Hilbert:

While it would be easier and more straightforward to save plots into png files, it would definitely be more useful to save the plots in some format such that they will retain their interactivity when they are added to the html template.

I do think that these plots need to be made asynchronously and saved, just like the other preview image files. 

https://docs.bokeh.org/en/latest/docs/user_guide/output/embed.html