Notification tweaks

After some user feedback made it clear that most folks weren't aware that getting email and IM notifications for about-to-start events was an opt-in procedure, I tweaked the code to so that newly created users now start out with all notifications turned on.

If you signed up for an account prior to Feb. 24, you still need to opt in to receive email or IM notifications. Visit your preferences page, which you can find under the toolbar link, and then tick the appropriate checkboxes and hit the "Update contact info" button. Similarly, newer users may opt out by visiting the same page and un-ticking checkboxes to their pleasure.