Njdupoux/proxy bug fix

Fix for proxy bugs stemming from improper html sanitization

In order to prevent improper html being sent back from the grader and proxy due to lack of escaping of characters such as "&", I've cleaned up the html formatting code to cleanly sanitize the html feedback response received from the grader. This includes proper html escaping. 

I've also written a few units tests to test the sanitization, and added some documentation 
Yeah I found that those modifications - changing <br> to <br/> and using single quotes - were required to make things work. Otherwise, I'm assuming the xml parser got angry. I did most of my testing using a real setup with the db_class_push_dev queue, and found that it all worked with the modifications I have here, and errors occurred otherwise (With the <br> and double quote issues specifically, a message along the lines of "Improperly formatted feedback" was shown). 

Thanks for the feedback @jbau :)