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?
Enter one or more sample responses that you've created, and then specify the options that you would choose for each criterion in your rubric. Note that you must add your rubric to the Rubric tab before you can complete this step.