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?
Students learn to assess responses by scoring pre-assessed sample responses that you provide. Students move to the next step when the scores they give match your scores. Note that if you add this step, you must also add a peer assessment step. This step must come before the peer assessment step.