Add history

[begriffs]

People are liking this app and want the history feature so they can later overlay people's reactions onto video recordings of their talk. The video integration stuff is way out of scope for now obviously, but the history would be good.

I'm thinking provide a graph of the results, and downloadable CSV (or JSON) for consumption by programs.

@dubharmonic do you want to help with this?

[ericsorenson]

Yes, definitely! You can assign this one to me.