Org storage must not duplicate notes

[jbiason]

If downfav crashes while saving Org notes, restarting the app would duplicate the entries in the org file.

To prevent that, we must start writing in a different file (using the current one, if it exists) and rename/move back to the proper place in the end, if everything went fine.

Attachments are not a problem, since they are stored in the filesystem and, thus, can be overridden without issue.