Add a setting to disable katex rendering

[nsarrazin]

Describe your feature request

Most people probably want katex rendering by default but because we need to support all the different katex delimiters, sometimes you can have issues in sentences where it triggers when you don't want to. (stuff like php variables "$foo and $bar") for example. Easiest fix would be to just have a toggle for users

Implementation idea

Just an extra toggle in settings stored in the db, use $settings in MarkdownRenderer to know if you should skip the renderLatex transformation