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

routers/common/markup.go

@@ -31,6 +31,8 @@ func RenderMarkup(ctx *context.Base, ctxRepo *context.Repository, mode, text, ur
 	// for example, when previewing file "/gitea/owner/repo/src/branch/features/feat-123/doc/CHANGE.md", then filePath is "doc/CHANGE.md"
 	// and the urlPathContext is "/gitea/owner/repo/src/branch/features/feat-123/doc"
 
+	ctx.SetHeaderContentSecurityPolicyGeneral()
+
 	if mode == "" || mode == "markdown" {
 		// raw Markdown doesn't do any special handling
 		// TODO: raw markdown doesn't do any link processing, so "urlPathContext" doesn't take effect