Übunsgblätter are sorted incorrectly in tutor view

[tim-kuechler]

Description: Übungsblätter are sorted alphabetically instead the should be sorted by their number

How to reproduce:

1. Go the the details view of a Übungsgruppe.
2. Click on Punkteübersicht
3. Hover over the points of an student in the column Übungszettel

[ekzyis]

This sounds like https://github.com/muesli-hd/muesli/issues/28

This was fixed in https://github.com/muesli-hd/muesli/pull/71 even though I just noticed #28 is not closed yet. (Can a maintainer confirm that #28 was resolved and close it?)

So maybe this issue here can be solved in the same way? Maybe I can find some time in the next days to have a look at this.