Prompt people to renew instead of doing it for them

[strugee]

In the currently deployed version people's checkins roll over to the next week. This is unideal because people don't know about this, etc. and so don't show up. Instead Checkins Bot should PM people on Zulip and ask them if they want to keep their checkin group. If they say yes or whatever, then do it.

/cc @porterjamesj - I would've filed this on your fork, but I can't because forks disable Issues by default. It'd probably be nice to upstream your changes here, anyway, so folks can find what's actually deployed more easily.