Disable Exam 'Save Answers' button during saving

@jbau, please
* Closes #1903 

 I noticed while reading to understand context that on line 706 we do
```javascript
  var saveXHR = $.post("{% url 'courses.exams.views.student_save_progress' common_page_data.course.prefix common_page_data.course.suffix exam.slug %}",
                           {   csrfmiddlewaretoken: "{{ csrf_token }}",
                               json_data:JSON.stringify(userSelections),
                               onpage:$(location).attr('href')
                           });
```

It looked to me like that json_data assignment could just use the value saved in window.myData, but I didn't change it because javascript scoping rules can be tricky.