Add boxes frontend

On this branch I added the feature to generate multiple choice options on the front end. The user is able to generate an arbitrary number of MCOs, assign different types of labels, drag them around and remove them. Once the finalize button is clicked the checkboxes and labels are printed to the PDF. The locations of the checkboxes are saved in the database to be used later for the pre-grading feature.

Closes #7 (closed)