Improve sidebar chat experience in Google Sheets

[npinsker]

We used Google Sheets integrated chat this year, but it's a bit less than ideal because it lacks chat history, an important way to get people without context up to speed. A Discord-based solution is worse -- unless Discord can be embedded in an <iframe>, even the small activation energy to switch between channels means that it can't serve as a reliable communication channel. Could maybe look into existing stuff (Zulip? IRC?) or consider building our own simple chat service.

It would be nice to come to a stance on this issue -- either improve on chat tooling or just declare that Sheets integrated chat is good enough