Fix/vik/studio oe

Fix several issues with open ended assessment.

* Remove incorrect/correct message to students, and rely on the rubric for feedback.
* Have markdown in studio by default, so that clicking save does not cause an error.
* Remove locations from staff grading view if the original problem has been deleted.
* Fix https://edx-wiki.atlassian.net/browse/OEE-4?jql=project%20%3D%20OEE%20AND%20resolution%20%3D%20Unresolved%20AND%20priority%20%3D%20CAT-2%20ORDER%20BY%20key%20DESC where OE assignments would not show up in progress.
* Fix https://edx-wiki.atlassian.net/browse/OEE-3?jql=project%20%3D%20OEE%20AND%20resolution%20%3D%20Unresolved%20AND%20priority%20%3D%20CAT-2%20ORDER%20BY%20key%20DESC where students got unlimited attempts.
* Add in test for attempt count.