Fixes for MultipleChoiceOption and FeedbackOption API

This merge request fixes two things:

1. Deleting a FeedbackOption will now throw an error if it is coupled to a MultipleChoiceOption
2. Inserting a MultipleChoiceOption will now check the problem_id parameter to see if an actual Problem with the specified problem_id exists.

Fixes #51 (closed) and #55 (closed)

Note: It might be useful to merge !16 (merged) first, since I can write test code for this MR.

tests/api/test_exams.py

+import pytest
+
+from flask import json
+from zesje.database import db, Exam, Problem, ProblemWidget
+
+
+@pytest.fixture
+def add_test_data(app):
+    with app.app_context():
+        exam1 = Exam(id=1, name='exam 1', finalized=False)
+        db.session.add(exam1)
+        db.session.commit()
+
+        problem1 = Problem(id=1, name='Problem 1', exam_id=1)
+        db.session.add(problem1)
+        db.session.commit()
+
+        problem_widget_1 = ProblemWidget(id=1, name='problem widget', problem_id=1, page=2,
+                                         width=100, height=150, x=40, y=200, type='problem_widget')
+        db.session.add(problem_widget_1)
+        db.session.commit()
+
+
+def test_get_exams(test_client, add_test_data):
+    mc_option_1 = {
+        'x': 100,
+        'y': 40,
+        'problem_id': 1,
+        'page': 1,
+        'label': 'a',
+        'name': 'test'
+    }
+    test_client.put('/api/mult-choice/', data=mc_option_1)
+
+    mc_option_2 = {
+        'x': 100,
+        'y': 40,
+        'problem_id': 1,
+        'page': 1,
+        'label': 'a',
+        'name': 'test'
+    }
+    test_client.put('/api/mult-choice/', data=mc_option_2)
+
+    response = test_client.get('/api/exams/1')
+    data = json.loads(response.data)
+
+    assert len(data['problems'][0]['mc_options']) == 2