Fix edge case for ordering of athletes in printable attendance view

[silviot]

Currently the order of booked lessons is off on the printable attendance view when two overlapping slots are defined. Check out feb 12th.
This is the current state of the printable view for that date:

The order should be changed using these criteria:

• start time ascending
• duration ascending
• last name

Again, this is caused by two slots that were created by mistake for the same time period. Still, our ordering should follow the above rules, and mix slots if necessary.
It's a nice to have, but we don't need to worry about this right now.