Correctly show 404 page on /sitemap.xml on multi-user instances

Sitemaps are only meant for individual blogs right now, so instead
of invalid HTML getting returned, we'll properly catch the issue and
show a user-friendly "not found" page.

Fixes #941

posts.go

@@ -314,6 +314,8 @@ func handleViewPost(app *App, w http.ResponseWriter, r *http.Request) error {
 	// Display reserved page if that is requested resource
 	if t, ok := pages[r.URL.Path[1:]+".tmpl"]; ok {
 		return handleTemplatedPage(app, w, r, t)
+	} else if r.URL.Path == "/sitemap.xml" && !app.cfg.App.SingleUser {
+		return impart.HTTPError{Status: http.StatusNotFound, Message: "Page not found."}
 	} else if (strings.Contains(r.URL.Path, ".") && !isRaw && !isMarkdown) || r.URL.Path == "/robots.txt" || r.URL.Path == "/manifest.json" {
 		// Serve static file
 		app.shttp.ServeHTTP(w, r)