Team notifications

[MitsukaiDokuro]

1. When members join/leave, team settings change, or similar - put a system message in the teams chat to let other members know.
2. Add finer control about notifications from team chat. It bugs me that I always have lots of unread messages when I open my profile page, but when I check it's always only from the team and no private message.

[nanaya]

for the number 2, there's already option for team chat notification in the user settings page...