Screenshot tests for PDF generation

[framebassman]

• [x] Set up new CI service (semaphore)
• [x] Implement route for generation PDF preview / example
• [x] Implement base BackstopJS project for testing
• [ ] Run it on Semaphore CI
• [ ] Compare example screenshot with actual

[framebassman]

Unfortunately BackstopJS does not allow to test generated PDF. So close this issue as 'wont fix'