Link to GCP storage for files

WARNING: We need to reflect in the webpage that these won't be behind a
login.

GitLab Issue #2

symposion/proposals/models.py

@@ -248,5 +248,4 @@ class SupportingDocument(models.Model):
     description = models.CharField(max_length=140, verbose_name=_("Description"))
 
     def download_url(self):
-        return reverse("proposal_document_download",
-                       args=[self.pk, os.path.basename(self.file.name).lower()])
+        return self.file.url