Optional Visualize individual aggergation per key plots as boxplots

Right now the plots at `/individual-aggregation-per-key` are bar charts showing (I think) the total duration spent on each task prefix.

I'm debugging a problem right now where there's unexpected variation within a task prefix. If this chart could (optionally) show the values as a boxplot ([bokeh docs](https://docs.bokeh.org/en/latest/docs/gallery/boxplot.html)) it'd be easier to verify that this is indeed the case.

cc @quasiben if you have thoughts.