Add KaTeX rendering to Markdown. (#20571)

This PR adds mathematical rendering with KaTeX.

The first step is to add a Goldmark extension that detects the latex
(and tex) mathematics delimiters.

The second step to make this extension only run if math support is
enabled.

The second step is to then add KaTeX CSS and JS to the head which will
load after the dom is rendered.

Fix #3445

Signed-off-by: Andrew Thornton <art27@cantab.net>

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

modules/setting/setting.go

@@ -344,10 +344,12 @@ var (
 		EnableHardLineBreakInDocuments bool
 		CustomURLSchemes               []string `ini:"CUSTOM_URL_SCHEMES"`
 		FileExtensions                 []string
+		EnableMath                     bool
 	}{
 		EnableHardLineBreakInComments:  true,
 		EnableHardLineBreakInDocuments: false,
 		FileExtensions:                 strings.Split(".md,.markdown,.mdown,.mkd", ","),
+		EnableMath:                     true,
 	}
 
 	// Admin settings