Fixed MATLAB Run Code bug with answer boxes.

LMS-2807

There was a 1-click delay whenever the "Run Code" button is clicked. And error messages also appear to be mis-formatted when "Run Code" is clicked, with HTML tags appearing instead of the relevant color text.

Added background task to get problem response from matlab queue.

@adampalay no this is for xqueue stub server which send grader response back to lms after 3 or 4 seconds.