Enh: tool to fix up missing attachments

[sebbASF]

Further to #501 and #502, existing databases will generally need to be refreshed in order to recover the missing attachments. This cannot safely be done by reloading from the source, because that may change the Permalink, especially for earlier mails which may have used a different algorithm.

The tool could reparse the source (from the raw mail) and generate an updated list of attachments. As a cross-check, any existing attachments should be found in the updated list; any missing ones can then be added.