New blog events default to today

When you make a new event on the blog, you can set the Start Time and the End Time. But when you have made the event, suddenly it's the top event on the frontpage. It turns out there's another, optional, time you can set, called "Calendar date", and if you don't set it, it fills in today.

We should make it fill in the same date as Start Time instead. Or make it leave the optional thing empty, and use the Start Time if Calendar date isn't filled in.

Long term goal should be having drupal copy the start date into the calendar date.