Bug: import-mbox.py should not default invalid dates to 'now'

[sebbASF]

It makes sense for archiver.py to default invalid or missing dates to now, because the archiver deals with live messages.

However it does not make sense for the importer to do so.

[sebbASF]

However dropping mails that don't have valid dates does not seem right either.
One possible approach would be to use the last timestamp from the Received headers, as that should be closest to when the message was sent.
That approach would also work better for the archiver.