Stop raising exceptions when login redirect URL is invalid

Previously, userena would raise an unhandled `NoReverseMatch` exception if the value of `next` in a login form was set to something invalid. This is because `django.shortcuts.redirect` was being used to perform the redirection, which can accept a URL name (which it will attempt to reverse) as well as a URL path. This PR replaces the url of `redirect` with an `HttpResponseRedirect`.