Adding/Deleting options in a multiple choice question introduces offset in given submissions

When a multiple choice question gets modified while there are already given submissions, the answers might get offset depending on whether options were deleted or added and therefore end up being wrong.

Expected results:

• A warning should be shown that answers might get offset and therefore end up wrong
• The offset for given answers should be also modified accordingly to ensure correct mapping after adding/deleting options
• Editing of questions should not be allowed when answering has started and editing would falsify given answers

Steps to reproduce:

1. Create feedback form
2. Add multiple choice question with at least 3 options, e. g.
   • Option A
   • Option B
   • Option C
3. Submit feedback with Option B selected
4. Observe that submission has submitted Option B in multiple choice question
5. Delete Answer B from multiple choice question
6. Observe that the same submission from step 3 now has seemingly submitted Option C in multiple choice question

Note that the issue also works in the other direction when an option gets added.

Might be related to MDL-17729.