Allow a slider to scanning through history of organizers

[patcon]

Demo of potential look of slider:
https://civictechto.github.io/people-list-parser/slider-demo.html

(sorry, bootstrap is a bit broken on that demo page. also, no functionality yet, just demo of input)

The idea would be that it would default to current date when page is reloaded, but then you could scroll back in time and see the list of organizers change. This interaction would be generated by the checkin data spreadsheet which could be updated in an ongoing way via regular interactive slack messages to organizers.