Implement group notices

[lislis]

@sareg0 came up with the idea of group notices.

Groups can post short notice updates on its meetings. Think of things like: the coaches are sick -> meeting cancelled, group goes to a meetup, group meets extra-ordinarily for cake.

Think of a bulletin board on the group#show that shows the last 2 or 3 notices. It also gives outside people an impression of how active the group is.

Optionally, group members can get an email when a new group notice is posted.

[sareg0]

🍰 🍥 🎥 🥇

I think this is a nice way to post updates for people who aren't in the mailing list but can see our public page. We don't have to rely on Twitter that way, and there is no doubt when the message is for because we could put a date on it and everything.

I'm not sure how much effort it could be but even putting an expiry on the message might be good. Even just posting the meeting is happening as normal is reassuring for people.