Allow descending order of items

[th3hamm0r]

Currently, the model can only be sorted ascending. With that change, you can now use the "-" prefix, e.g. on the model's Meta, to use descending order. For example:

from django.db import models
from wagtailorderable.models import Orderable

class YourModel(Orderable):
    ...
    class Meta:
        ordering = ["-sort_order"]

This PR also includes the following improvements:

• Try to use the model's default ordering from the Meta class, if the Modeladmin does not define one.
• Cleanup: Removed get_ordering(), since the OrderableMixinMetaClass already ensures, that the sort column is part of the ordering.