Pushed a big change to enable group chat.

This kinda seems like feature creep, but looking at successful forums, I think many of them have an irc (or a fucking discord) on the side. Or you resort to an adhoc chat post. So I think it's fairly important. It's not very complicated, either.

It's not on by default, and can be enabled on a per group basis.

But does it fedi? Obvi! Well, within reason, and with certain caveats.

It's based on the ChatMessage type, addressed to the group, and federated via Announce/Create/ChatMessage like other group activities. So nothing special.

Honk required a small fix because it wasn't expected chats to be announced. Not sure how other software would react. The fix was pretty simple and obvious, just not something I anticipated. The tricky part is getting addressing right and replying to the group, not only the poster.