Hardcoded but abstractable C3.js-based viz

- Card index versus date with size of dot indicating pass rate.
- Histogram of pass rates!

TODO: select which decks to consider (also for CSV export) and which field
to treat as the key. Hardcoded currently to my RTK deck.

TODO: make the crosshairs illuminate the datapoint better. I had to
disable automatic resizing of the hovered-over dot because it wouldn't
remember the dot's custom radius :(.
Skips viz on reviews of deleted cards
Mouseover and mouseouts for indicating data: meh

Meh because the tooltip *isn't* attached to simple mouseover/out. The
tooltip stays there while the mouse moves over no-point-land. I could put
some point indicator in the tooltip callback but then I don't know how to
reset the point's indicator after the tooltip leaves it.
Very interesting plot of days known/# lapses

Trying to hide the stupid "index" variable that lets me put interesting
things in the tooltip.

Also trying to get the tooltip to not be asanine. It appears to track the
nearest x position: why??? Why not track the closest point in Euclidean
distance?
Four visualizations. Going to make general.

Submitted dupe issue to C3js about broken mouse-tracking in one x/y pair
scatter plot.
Adding responsive UX for viz
D3 selectors for model choice is working

Time to hook it up to vizualization
Visualize button all connected!
Visualizations in, with some explanatory text.

Only some slight testing.