Makes changes proposed in #46 (needs testing)

Run these MySQL commands:
    rename table ratings_team to ratings_group;
    rename table ratings_teammembership to ratings_groupmembership;
    alter table ratings_groupmembership change team_id group_id int(11) not null;
    alter table ratings_groupmembership add override tinyint(4) not null default 0;
    alter table ratings_group add team tinyint(4) not null default 0;
    update ratings_group set team=1;
    alter table ratings_message change team_id group_id int(11) null;
    alter table ratings_alias change team_id group_id int(11) null;