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?
In this assignment, you can include steps for student training, peer assessment, and self assessment. Select the steps that you want below, and then drag them into the order that you want. If you include a student training step, it must precede all other steps.