add fix_grades management command

Shows any lost grades in a proctored and randomized edx course. Has a
dangerous db/grade-modifying option that will attempt to automatically
repair the issues:

--fix-it-for-real-i-mean-it
reset choice if current choice was removed from XML
reset choice if not viewed and is suspended

Randomize module will select another choice if the problem is suspended
and hasn't been viewed by the student. If the student has already viewed
the problem then we can't reset in case they got the problem correct -
only a reset can assign another problem in this case.