Moved cookie unset to index level

So that it can be applied on any given page load
Computer today's date once
Made createEmailContent() public

Since this method will be called from outside the object to produce a
preview of an email message.
Added reminder system

You now see a “remind” link next to the names of tomorrow’s
participants. This allows you to preview and send a reminder email, as
defined by the email_remind element in the experiment file.