RSVP as an authenticated user (centralized case)

[elmariachi111]

As a logged in user I can hit an RSVP button to "sign in" to an event. A server side function writes that rsvp data (the user's chosen public profile) to a Slack channel (assuming we're only managing coding earth events here) (later: have an attendee list)

At the same time we're setting a simple local storage flag that an user has signed in (which easily can be "hacked", but that's okay). if that local storage flag is set, disclose the online attendance URL. (later: x-check with the backend and build a unique join link or use a ledger based rsvp process)