Add a rake task to re-extract any missing attachments

You can see the number of emails that would be reparsed
by doing:

  bundle exec rake temp:reextract_missing_attachments

To actually reparse the incoming emails and rextract
attachments for any that were missing, you would do:

  bundle exec rake temp:reextract_missing_attachments[commit]

(In fact, the 'commit' can be any non-empty string.)