Enhancement: "respond-by" date

When scheduling events we need people to respond by a certain time to be able to organize things. How much time is needed depends on the event type - might have to book a room, or just send out an email with a Zoom link - so this should be configurable per event. It could probably be defaulted to the first time option -24h.

For additional motivation, a follow-up enhancement suggestion will be:

> Thanks for adding this wonderful option for setting the "respond-by" date for a Newdle! Now that we have it, could Newdle remind participants to fill in the Newdle, say 24h before the respond-by date, for those that didn't respond yet? It would be *so* much nicer if a computer did it, reliably, instead of me, unreliably!

😄

Thanks for considering!

The issue names no files, tests, or entry points. Start by mapping how event options are stored and scheduled, then determine where a per-event respond-by value and its default belong. Done means events can configure the deadline independently, with the proposed default applied when none is set.

Written by the indexing model from the issue text.