Support customizing generated ordering e.g. to sort nulls last

[stevelacey]

Support sorting nulls last via overriding get_sort_field, not my favorite syntax, but any approach involving view configuration seem to require we either make assumptions or add another view configuration attribute.

    def get_sort_field(self, name, descending=False):
        if name == "my_field" and descending:
            return F(name).desc(nulls_last=True)
        return super().get_sort_field(name, descending=descending)

Where should the reviewer start?

List APIs.

Why is this important, or what issue does this solve?

Sorting nulls last via the sort query param is not really possible without this.

This is a modification of #73.

Tasks

• This PR increases test coverage
• I didn't add anything to the docs because I hate it