MIGRATION: Add multiple selection and rationale to multiple choice embeddables
Updated models and extraction for Multiple Choice multi selection and rationale
Report UI for multiple-select and rationale on multiple choice

Attach check marks to the actual items, so teachers can then see which student choices
were correct on a multiple-select question with correct answers.
Learner Report UI for multiple-select and rationale on multiple choice
Fix bug in extracting multiple choice when only the rationale text has changed