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/Exam.jsx

@@ -733,16 +733,6 @@ class Exams extends React.Component {
         )}
         {props.problem &&
           <React.Fragment>
-            <div className='panel-block'>
-              {!this.state.editActive && <label className='label'>Feedback options</label>}
-            </div>
-            {this.state.editActive
-              ? <EditPanel problemID={props.problem.id} feedback={this.state.feedbackToEdit}
-                goBack={this.backToFeedback} updateCallback={this.updateFeedback} />
-              : <FeedbackPanel examID={this.props.examID} problem={props.problem}
-                editFeedback={this.editFeedback} showTooltips={this.state.showTooltips}
-                grading={false}
-              />}
             <div className='panel-block mcq-block'>
               <b>Auto-approve</b>
               <div className='select is-hovered is-fullwidth'>