notifications

[olliestanley]

We need more notification messages for things like:

• [ ] Players being told when they're invited to a group
• [ ] Players being told when they're kicked from a group
• [ ] Group members with MANAGE privilege being told when they're invited to join a parent organisation
• [ ] Group members being told when a player leaves the group
• [ ] Group members being told when the group becomes a child or parent organisation
• [x] Login message of the day etc
• [ ] On-login reminder of pending group invitations etc