Implement ordering

this MR finally implements ordering using django filters OrderingFilter, see https://django-filter.readthedocs.io/en/main/ref/filters.html#orderingfilter

it's not yet a super-intuitive solution, as the ordering is hidden in the filter menu. but at least, we have it implemented 😉

it looks like this:

closes #186 (closed)

ping @beate.geyer @csteger