implement group chat invites and bans

[williamhorning]

1. to add people to group chats, you must invite them
2. invites should be able to optionally expire
3. inbox notifications should be redone to allow you to accept/reject invites
4. inbox notifications should be extensible so we can add new ones in the future
5. you should be able to ban people from groups
6. bans don't remove you from the group, they display a ban message instead
7. bans should be able to expire
8. ip bans should also be possible