Implement PDF export for graded Missions and Trainings

Resolves #260 
If you only add PreferableItems here, I think only new courses will have this item in settings, existing courses will not be able to use this new feature. we might need a rake script to add this preference to all existing courses.

check the function <code>populate_preference</code> in course.rb, this function only be called when a course was created.