Moved meeting announcements and notes

I’ve renamed the “Meeting Notes” category to just “Meetings,” to also cover meeting announcements… which we seem to like to annotate with notes in the same topics, rather than starting new ones for the notes. So we can keep doing that, but I’ll make announcements in the Meetings category rather than the Chat category, and all discussion of meeting times etc. should also happen there. I think I’ve moved all of the old stuff of this kind from Chat to Meetings.