Add ability to find by quiz_id
Update gems
Add slides to quizzes as a relation
Create project_submissions for each quiz_submission_content
fixing MR for submissions per slide
fixing intro to R for one submission per slide
re-org quiz for introt to R
Merge branch 'mark-reconstruct-quiz-submissions' of https://github.com/teamleada/teamleada.com into tristan_single_submission_per_page
adding slide to quizzes in intro to r
adding slide to mh old one
adding slide to quizzes in 005 titanic bootcamp
fixing flu trend quizzes to use slides (and splitting per slide)
fixing mr 016 to have slide for quizzes