Update messages in authoring UI

* Update messages in authoring UI
* Remove unsupported assessment configurations
* Set default dates to something reasonable.

@srpearce These are the messages from the spreadsheet you created as of Friday morning.  Could you look over these and let me know if there's anything you'd like me to update?
Just to make sure: this is going to be below the text area where instructors can enter XML with training examples.  Is this still the label you want?