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_feedback.py

+import pytest
+
+from flask import json
+
+from zesje.database import db, Exam, Problem, ProblemWidget, FeedbackOption
+
+
+@pytest.fixture
+def add_test_data(app):
+    with app.app_context():
+        exam1 = Exam(id=1, name='exam 1', finalized=False)
+        db.session.add(exam1)
+
+        problem1 = Problem(id=1, name='Problem 1', exam_id=1)
+        db.session.add(problem1)
+
+        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 mco_json():
+    return {
+        'x': 100,
+        'y': 40,
+        'problem_id': 1,
+        'page': 1,
+        'label': 'a',
+        'name': 'test'
+    }
+
+
+'''
+ACTUAL TESTS
+'''
+
+
+def test_not_present(test_client, add_test_data):
+    req = mco_json()
+
+    result = test_client.put('/api/mult-choice/', data=req)
+    data = json.loads(result.data)
+
+    fb_id = data['feedback_id']
+    problem_id = 1
+
+    result = test_client.delete(f'/api/feedback/{problem_id}/{fb_id}')
+    data = json.loads(result.data)
+
+    assert data['status'] == 401