Expiry dates

OK, expiry dates on Xchanges are now working properly. (I think -- please test by posting some fake Xchanges that will automatically expire!)

Here's how it should work:

• Posts with no expiry date will never expire (duh!)
• If a post has an expiry date, you will receive a warning e-mail when the expiry date is reached.
• The day after the expiry date, the Xchange will no longer show in the directory.
• 7 days after the warning e-mail, the Xchange will be permanently deleted. (This happens silently, no e-mail to the user).
• If you edit the Xchange during the 7-day period, you can change or remove the expiry date and the Xchange will be reinstated to the directory.