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?
Replace this text with your own sample response for this assignment. Then, under Response Score to the right, select an option for each criterion. Students learn to assess responses by assessing this response and comparing the options that they select in the rubric with the options that you specified.