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/errpage.go

@@ -40,6 +40,7 @@ func renderServerErrorPage(w http.ResponseWriter, req *http.Request, respCode in
 	if acceptsHTML {
 		err := templates.PageRenderer().HTML(outBuf, respCode, tmpl, ctxData, tmplCtx)
 		if err != nil {
+			log.Error("Failed to render error page template %s: %v", tmpl, err)
 			_, _ = w.Write([]byte("Internal server error but failed to render error page template, please collect error logs and report to Gitea issue tracker"))
 			return
 		}