Heavy course validation on front-end

[canstudios-nicolaw]

On every preview/export/download, the front-end is running a function called validateCourseContent. This function is looping through all pages/articles/blocks/components in a course to ensure that they all have children, each item is fetched from the back-end individually. This is inefficient as it can generate hundreds of requests and cause long delays (8+ seconds) on larger courses.

Remove the validateCourseContent function and replace it with back-end validation that happens as part of preview/export/publish. The user should still get an error modal with the same information if the course is invalid, but this should be a more efficient approach as we will have access to the db.