Team participation

Allow to setup a contest to team participation. ("allow_team_participation: true")

A user starting the contest is offered to enter an additional logincode for his team mate.

A shadow-participation is created for the team mate. It references the main participation.

The resulting contest export could contain one line per team or two lines per team. (Inquiry pending)

Optional: The team mate can see the participation as if it was his own. (?)