Lesson preview

A course owner now has the ability to preview how a formatted lesson will look to a potential student.

* able to preview during editing and creation of lesson
* can either publish straight from preview or go back to edit some more
* uses /api/utility/markdown endpoint with a POST request
* uses generic error template if something goes wrong trying to produce preview
 
note: its not done with modals, the initial commit was made before I realized that modals were not the way to go.