Adds a thin wrapper for DOMDocument to simplify and align usage within
all areas of BookStack.
Also means we move away from old depreacted mb_convert_encoding usage.
Closes#4638
Was causing things like emjoi within script content to be somewhat
mangled. Instead we force UTF8 only parsing via XML declaration.
Added test to cover.
For #2923
Fixes a strange issue of HTML tags within script tags being malformed
when part of the HTML custom head content due to the PHP parsing we do.
DOMDocument seemed to cause this upon load.
Adding LIBXML_SCHEMA_CREATE to the ->loadHTML call seems to fix this but
not really sure why. Doesn't seem to cause further issues though.
Tested with multiple scripts and styles and comments and meta tags.
- Also added new testing class to cover.
- As part of testing, added new folder within tests to house setting
specific tests.
For #2914
