Printing in firefox is broken due to flexbox #1211

Describe the bug
Printing in Firefox is truncated and only prints the first page of content.

The is likely due to flexbox and this Firefox bug.

A workaround would be to apply display: block to the main content elements in the print CSS.

Steps To Reproduce
Steps to reproduce the behavior:

1. Open Firefox
2. Go to any content page in Bookstack that is long enough to print on multiple pages
3. Print the page
4. Only the first page prints

Expected behavior
The entire document should print.