Toggling pregrading and Identifying blank solutions

This merge request does two things:

• Automatically identify blank solutions (either multiple choice or not) and set a feedback option named 'blank' for each one.
• When creating an exam, there are three options to specify which questions should be graded automatically.
• Pre-grade a question again if a submission is reuploaded and is automatically graded (by 'Zesje')

The grading policy is specified in the following way:

• 0/set_nothing: Manually approve everything
• 1/set_blank: Automatically approve blank solutions
• 2/set_blank_single: Automatically approve blank solutions or solutions with one answer

Add a new field to a problem in the database with this number.

For a more detailed description, see #61.

Closes #33 and #61.

client/views/Graders.jsx

@@ -26,7 +26,9 @@ class Graders extends React.Component {
         this.props.updateGraderList()
       })
       .catch(resp => {
-        Notification.error('Could not save grader (see Javascript console for details)')
+        resp.json().then(e => {
+          Notification.error(e.message)
+        })
         console.error('Error saving grader:', resp)
       })