Check Book Readiness on Submit

[jonthegeek]

When a user submits new times for a book, that MIGHT be enough to put it over the "minimum of 5 interested people" threshold to start a new club. It would be cool if new submissions could trigger an API to check for that.

We'd need to store facilitator volunteers before this (likely separate ticket, will edit and link to it once it exists).

We'd also want to have an automated process for attempting to choose a time + posting about it, and to make sure that process wouldn't be annoying with repeated submissions (probably limit how quickly the recheck would post to Slack or something along those lines).