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/components/PanelMCQ.jsx

@@ -90,9 +90,8 @@ class PanelMCQ extends React.Component {
           nrPossibleAnswers: 2,
           chosenLabelType: value
         }, () => {
-          this.updateNumberOptions()
           let labels = this.generateLabels(this.state.nrPossibleAnswers, 0)
-          this.props.updateLabels(labels)
+          this.updateNumberOptions().then(() => this.props.updateLabels(labels))
         })
       } else {
         this.setState({
@@ -116,7 +115,7 @@ class PanelMCQ extends React.Component {
 
     switch (type) {
       case 1:
-        return ['T', 'F']
+        return ['T', 'F'].slice(startingAt)
       case 2:
         return Array.from(Array(nrLabels).keys()).map(
           (e) => String.fromCharCode(e + 65 + startingAt))