feat(subscription): Add comments for subscriptions

Close #3

tests/test_nightly_tasks.py

@@ -39,6 +39,7 @@ def test_subscriptions(db_session):
             num_adult_meals=2,
             num_children_meals=1,
             num_small_children_meals=0,
+            comment="test",
         )
     )
     db_session.commit()
@@ -54,3 +55,5 @@ def test_subscriptions(db_session):
     assert not event1.subscriptions_applied
     assert not ignore.subscriptions_applied
     assert event2.subscriptions_applied
+    assert event2.registrations[0].comment is not None
+    assert "test" in event2.registrations[0].comment