Implement PDF export for graded Missions and Trainings

Resolves #260 
@yangshun The specificity for `label` and `p` is needed to override the following styles in `custom.css.scss`:

https://github.com/jiunwei-moe/coursemology.org/blob/assessment-printing/app/assets/stylesheets/custom.css.scss#L33
https://github.com/jiunwei-moe/coursemology.org/blob/assessment-printing/app/assets/stylesheets/custom.css.scss#L87

(Alternatively, the offending lines in `custom.css.scss` can be removed.)

The `.comment` selector is needed as a result of the new markup for rendering code in `_code_for_printing.html.erb`. Otherwise, comments would be rendered in a monospace font as they are now embedded in a `cos_code` `div`.

More generally, these rules to enforce a plain `sans-serif` font family are needed to work around a bug in `wkhtmltopdf` that occasionally blanks out text rendered in the `Lato` font family.

I will push out a commit to use nested selectors shortly.