Fixing bad dates in migration

[sebb <se...@gmail.com>]

There are several issues with dates in the original PonyMail.

For example, mails are ordered chronologically by the Date field on
emails, even though that can be wildly wrong. At least with mod_mbox
the worst that could happen was the mail was shunted to the end of the
current month.

Also the displayed date is not the same as the Date: header.

Further, for a while date calculations used localtime rather than gmtime.

The 'date' fields are not consistent.