Dashboard Tracks Date User RSVP's and Check-In

This will be useful for a dashboard analytics page so that each hackathon can see previous year's stats if they do a database dump. I will flesh out this story once I have more of an idea of exactly how we are going to do this.

The issue does not name files, tests, or an entry point. First clarify the dashboard analytics requirements, including which RSVP and check-in dates must be retained across database dumps; the work is done when the data model and previous-year reporting behavior are agreed.

Written by the indexing model from the issue text.