Staff information about duplicate ORA submissions

Pulled out for review, please don't merge yet.

The open response assessor checks for duplicate submissions, and doesn't let staff grade anything that is a duplicate.  So if you make a submission and it is a duplicate, you will have to wait for up to 30 minutes for a grade to come back, and have no warning of what is happening.

Staff have expressed some frustration over this, because it results in them submitting something, going to the staff grading view, and not seeing anything to grade.  They then think that it is a bug, when it is not.

This PR gives them information about duplicate submissions.

Other ways to address the same problem:

* Give staff a way to reset submissions (ie, make reset state in the instructor dashboard work for ORA and capa, not just capa)
* Have some more info/warnings in edX101 (I don't think this is sufficient).
* A popup in studio with the information?

Pulling this out so that we can discuss how to best communicate this information.  @marcotuts your input would be great here, as would yours, @stroilova .

Here is a screenshot of how it looks (see the very bottom):

![staff_warning](https://f.cloud.github.com/assets/913340/1089989/3951658a-164e-11e3-9526-13e9176cbb5c.png)


Another issue is that when a submission is shown to the staff to grade, it is "checked out" on the backend and removed from the grading pool.  If there is no response for 30 minutes, the submission is put back into the pool.  When staff view a submission, and then hit back, they are surprised about why the submission "disappears", but it will in fact return in 30 minutes.  Add some messaging around this:

![selection_009](https://f.cloud.github.com/assets/913340/1089994/454deaf2-164e-11e3-90a1-414f1b907554.png)