Implement PDF export for graded Missions and Trainings

Resolves #260 
Wow it's really called `molokai` in Coursemology according to https://github.com/Coursemology/coursemology.org/blob/development/app/assets/stylesheets/custom.css.scss#L713-L728.

But on CodeMirror it's https://github.com/codemirror/CodeMirror/blob/master/theme/monokai.css. The colours are off too. Perhaps Molokai is a new theme on its own.