Default notifications for groups and DMs

This is a chronic issue

**User Problem**: Users expect DMs and mentions to notify them by default - but not random conversation in group chat rooms.
**Defect:** Public chats notify for everything by default, encouraging you to mute the app
**Suggested plan:** Switch the defaults to mute public rooms by default, and grandfather in the old push behaviour. Avoid getting sucked into "must fix notifications from first principles".