Added some validations for the user registration

Added validation: only one team per course in a tournament
Added validation: the same athlete can't be inserted twice in the same team
Added validation: one athlete can only belong to one team in a tournament
When a manager is registered in a team it is added to the as an athlete also
Added the gem bullet
added eager loading to some associations
added 'New Team' to the Team's show page
alphabetically organized gems in Gemfile