Implement PDF export for graded Missions and Trainings

Resolves #260 
@jsyeo You're right. I'm removing the redundant require statement and moving the shared code between `GradingsController` and `AssessmentsController` to a new `GradingsSummaryBuilder` module in `app/modules` instead. The shared code was previously placed in `GradingsHelper`, which doesn't follow the intended purpose of helpers in Rails.