Standardise PDF export size and allow configuration between a4/US-letter #995

Describe the bug
I'm using wkhtmltopdf and the output PDF is not 8.5x11. Any ideas how to change that now, or can it be something in the user-edited styles to be added? Before we switched to wkhtmltopdf the document size was 8.5x11.

Steps To Reproduce
Steps to reproduce the behavior:

1. Export document with wkhtmltopdf

Expected behavior
document size 8.5x11

Screenshots

Your Configuration (please complete the following information):

• Exact BookStack Version (Found in settings): v0.22.0
• PHP Version: 7
• Hosting Method (Nginx/Apache/Docker): Nginx