Auto insert year in event URL

[jace]

When creating an event users always forget to include the date in the URL, making it difficult for them to do the same event the following year.

Hacknight should enforce names in the form of <year>-<name>, automatically inserting the year if the user left it out (or left the field blank) but not adding it again if the user already included the year in the name.

The year is automatically updated if the user changes the event's dates. Since Hacknight now tracks past URLs, this will not break links.