Unify bokeh plot theming

[ml-evs]

After watching people make blocks on other systems, I notice how inconsistent fonts and bokeh style is. As part of #330, we should also unify this theming with some good defaults.

[jdbocarsly]

The current way to (hopefully) stay consistent in styling is to generate the plot with:

self.data["bokeh_plot_data"] = bokeh.embed.json_item(layout, theme=bokeh_plots.DATALAB_BOKEH_THEME)