#1534 lockout between exams

@jrbl 
This is the code that checks if there has been a recent submission, and doesn't allow the display of the exam/pset/quiz/foo if there has.  Also checks across the contentgroup.

I imagine we should pull this in _after_ tonight's release. (sprint-29-pre-1).

@sefk.  There is a column addition for the lockout period, which defaults to 0.  Not 100% sure if a data migration is the proper way to set things up for the DB course, since it would be pretty hardcoded lto ook for exam.course="db" and set the value if true, but maybe it is, especially if that migration has a short lifetime due to a burndown.  If it is, I can create one, and it would make sense for that to be the _last_ migration before the burndown.