Allow regrading of submitted answers

[dlglin]

A commonly requested feature is to be able to modify a question and then retroactively regrade student's previous submissions. It should be possible to create some version of this if we are using the standalone renderer.

The one thing that I can't see a way to handle would be if a question was edited and answer blanks were added or deleted. In this case the past answers would not match with the answer blanks in the new question. If we were to implement such a feature, we would have to make it clear that it won't work if the number or order of answer blanks is changed.