There are more ShiftAccountEntry with is_solidarity_used=True than there are SolidarityShift with is_used_up=True

[Theophile-Madet]

At the time of writing:
SolidarityShift.objects.filter(is_used_up=True).count(): 218
ShiftAccountEntry.objects.filter(is_solidarity_used=True)s.count(): 225

If I understand the system correctly, both number should be the same.

Overall, I think the code for the solidarity shift system could be simplified a lot.