Make resolve_url support relative URLs

This fixes redirecting to relative URLs with django.shortcuts.redirect

Supporting relatives url is kind of a controversial topic. But it seems that the choice to support relative URLs has [been made](https://code.djangoproject.com/ticket/987), and [there's code to support it](https://github.com/django/django/blob/636860fbfb4e34e08749d4576bc9571028150526/django/core/handlers/base.py#L24).