Fix stock location structural check

[bloemp]

Exclude sub stock location items from preventing that a stock location can be switched to structural.

Closes #4087

[SchrodingersGat]

@bloemp thanks! Can you also make the same fix to the part category model:

InvenTree/InvenTree/part/models.py

Line 151 in d9e99e3

if self.pk and self.structural and self.item_count > 0:

[bloemp]

@SchrodingersGat modified the pull request to include a fix for the part categorie.
if self.pk and self.structural and self.partcount(False, False) > 0:
I've added both parameters to the partcount call, while not strictly needed... it seemed future proof (if anyone ever changes the default, the check still functions). But as a newbie to python, is this the best way to go?

[SchrodingersGat]

@bloemp looks good :)

[SchrodingersGat]

Thanks for your contribution @bloemp