compatibility issue with tidy 5.6 #1553

Describe the bug
There is a compatibility issue with tidy 5.6 that provoke the same issue mentionned in #610 but with every revision pages and without changing the editor.

It works well with tidy 5.4 and for some reason there aren't any version 5.6 for ubuntu (this why there are no problem with a bookstack installation on ubuntu yet).

Related to htmldiff #139

Current Behavior
from books/foo/page/bar/revisions only a few characters from each line are showing.

Steps to Reproduce
Docker linuxserver or install bookstack in a centos/redhat VM (with tidy 5.6) then :

• Create a book -> chapter -> page
• Add content and save
• Make an edit
• From the page go to More -> Revisions and click Changes for the edit

Screenshots
Example : https://ibb.co/37BJ5DF
Actually the hole page is like : https://ibb.co/BcCFvj4

Your Configuration (please complete the following information):

• Exact BookStack Version (Found in settings): 0.26.3
• PHP Version: 7.2
• Hosting Method (Nginx/Apache/Docker): Docker linuxserver and tried also on a redhat VM

Additional context
What I have tried :

• manual install on a redhat VM with nginx and apache -> same problem
• install with the installation script on ubuntu 16.04/18.04 VM -> no problem

On my redhat VM, I could fix the issue by :

• removing php-tidy
• installing tidy5.4.0 from http://binaries.html-tidy.org/
• installing php-tidy again
• rebooting