Use proper Django form to sort reviews

Submiting form on ProductReviewLis didn't preselect default value in
select element.

Fox example if I want sort by "Recency" click GO after submit select
option "Recency" should be set as selected="selected".

Closes #1511.
This commit is a squashed version of #1516.

@maikhoepfel moved the choices from a dedicated file into the form,
and changed the sorting to look at the form's cleaned_data.