add readonly mode for markdown property editor

src/Umbraco.Web.UI.Client/lib/markdown/markdown.css

@@ -20,6 +20,10 @@
 	width: 100%;
 }
 
+.wmd-panel .btn-toolbar-container[readonly] .btn-toolbar {
+	display: none;
+}
+
 /* unset the negative margin applied in button-groups.less to avoid flickering when hovering the button bar */
 .wmd-panel .btn-toolbar .btn-group>.btn+.btn {
     margin-left: 0;