Several multiple choice option fixes

Addresses issues mentioned in #29 (closed).

• Make migrations compatible with the migrations on master

Closes #29 (closed)

zesje/database.py

@@ -169,7 +169,7 @@ class MultipleChoiceOption(db.Model):
     label = Column(String, nullable=True)
 
     problem_id = Column(Integer, ForeignKey('problem.id'), nullable=False)
-    feedback_id = Column(Integer, ForeignKey('feedback_option.id'), nullable=False)
+    feedback_id = Column(Integer, ForeignKey('feedback_option.id'), nullable=True)
 
 
 class ExamWidget(Widget):