Use leaflet ID in image paths

This is a partial fix for #235, as it will prevent new uploads from
showing the wrong image. It doesn't fix old images / uploads.

Refs #235

electionleaflets/apps/leaflets/views.py

@@ -230,7 +230,7 @@ def done(self, form_list, **kwargs):
                         "Bucket": bucket.name,
                         "Key": file_path,
                     }
-                    new_file_name = f"leaflets/{file_name}"
+                    new_file_name = f"leaflets/{leaflet.pk}/{file_name}"
                     moved_file = bucket.Object(new_file_name)
                     moved_file.copy(copy_source)