Allow a slider to scanning through history of organizers

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](https://docs.google.com/spreadsheets/d/1LCVxEXuv70R-NozOwhNxZFtTZUmn1FLMPVD5wgIor9o/edit#gid=1061878392) which could be updated in an ongoing way via regular interactive slack messages to organizers.

Start with the linked slider-demo.html to understand the proposed interaction, then review the linked check-in data spreadsheet. Determine how the slider should default to the current date and how changing the date should update the organizer list. Done means the historical organizer lists change correctly from the spreadsheet data.

Written by the indexing model from the issue text.