Implement PDF export

Prettified PDF output for training_submissions

Added PDF export options

Changed disposition of PDF export

Implement PDF export for Missions

Add more options for PDF export

Respect pdf_comments for single-question Missions

Decouple views from params hash

Switch to wkhtml_binary_provider gem

Use sans-serif for PDF export

Add preferences for enabling PDF export

Fix stylesheet and formatting errors

Implement export of zipped PDFs

Fix missing line break

Refactor PDF export code