settings_um: Use live config

Use json/cfg for the usermod settings page.  Should fix issues
with outdated content when a new firmware is loaded.

wled00/data/settings_um.htm

@@ -13,7 +13,7 @@
 	function S() {
 		getLoc();
 		// load settings and insert values into DOM
-		fetch(getURL('/cfg.json'), {
+		fetch(getURL('/json/cfg'), {
 			method: 'get'
 		})
 		.then(res => {