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.

zesje/database.py

@@ -115,7 +115,8 @@ class Problem(db.Model):
     id = Column(Integer, primary_key=True, autoincrement=True)
     name = Column(Text, nullable=False)
     exam_id = Column(Integer, ForeignKey('exam.id'), nullable=False)
-    grading_policy = Column('grading_policy', Enum(GradingPolicy), default=GradingPolicy.set_blank, nullable=False)
+    grading_policy = Column('grading_policy', Enum(GradingPolicy), server_default='set_blank',
+                            default=GradingPolicy.set_blank, nullable=False)
     feedback_options = db.relationship('FeedbackOption', backref='problem', cascade='all',
                                        order_by='FeedbackOption.id', lazy=True)
     solutions = db.relationship('Solution', backref='problem', cascade='all', lazy=True)