Register for a free Luma event

Register the signed-in Luma account for a free public event by its slug. If the event asks for a phone number, a random one is used automatically. Only fails loudly on events that aren't free, aren't open for registration, or ask custom registration questions this script doesn't support. This signs you up as an attendee under your real identity, so confirm the event with the person you're helping before running it — cancelling afterward is a separate action.