refactor: markup render (#38864)

1. add missing CSP header to api & web render endpoints.
2. make jupyter render skip post-processors, nothing to process
3. make ShortLinkProcessor correctly validate URL schemes and respect
the CustomURLSchemes setting

modules/setting/markup.go

@@ -50,7 +50,8 @@ var Markdown = struct {
 	MathCodeBlockDetection []string
 	MathCodeBlockOptions   MarkdownMathCodeBlockOptions `ini:"-"`
 }{
-	EnableMath: true,
+	EnableMath:       true,
+	FileNamePatterns: []string{"*.md"},
 }
 
 // MarkupRenderer defines the external parser configured in ini