Refactor List, Next and Remainder into `simple_tag`. Django 1.4+

simple_tag() cannot handle filters in Django 1.3 so this causes 3 tests to fail. In reality though, the only scenario effected is `{% render_list list_var|filter %}`. That *previously* worked and filtering the template arguments to List/Next/Remainder never worked. (It was fixed in a58bfe04d9405cd5490dbae2bdbb8295afd0fa85 but that was never in a public release.)