Attachment support in the SQL backend

attachments working with the sql backend.

- SQL Schema for attachment files, ordering attachments by
  their name, not by the submission order (as earlier).
- Dot-Notation for attachments, where missing.
- convert existing attachments over from mongo -> sql

mediagoblin/edit/views.py

@@ -120,7 +120,7 @@ def edit_attachments(request, media):
             finally:
                 request.POST['attachment_file'].file.close()
 
-            media['attachment_files'].append(dict(
+            media.attachment_files.append(dict(
                     name=request.POST['attachment_name'] \
                         or request.POST['attachment_file'].filename,
                     filepath=attachment_public_filepath,