Groups with capacity zero cause problems

[nilsfriess]

To reproduce:

• Create a lecture
• Create tutorials
• Set the capactiy of at least one tutorial to zero
• Set the mode to "Praeferenzen"
• Let students choose preferences
• Click "Eintelung vornehmen" to assign students to tutorials
  In our case, this assigned students to a group with capacity zero and resulted in an internal server error (500) when visiting the lecture's overview page. It is still possible to visit the page where the tutorial can be modified directly. Moving all students that were assigned to the zero-capacity tutorial to a different tutorial or setting the capacity to a non-zero value fixes the 500 error.

So it seems that:

1. capacity zero is not correctly treated
2. having a N > 0 students in a group with capacity zero seems to cause an exception on the server (our assumption is that it's a classic divide by zero).

Arguably, having a group with capacity zero doesn't make sense so maybe the easiest solution is just to disallow having groups with zero capacity.