Do not inject a #app-navigation-toggle if there is already one

This allows apps to ship their own, as in some cases the #app-content
element does not exist on page load and therefore the injection fails
and the icon is missing afterwards.

Fixes #14956

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>

core/js/js.js

@@ -1192,7 +1192,9 @@ function initCore() {
 	setTimeout(resizeMenu, 0);
 
 	// just add snapper for logged in users
-	if($('#app-navigation').length && !$('html').hasClass('lte9')) {
+	// and if the app doesn't handle the nav slider itself
+	if($('#app-navigation').length && !$('html').hasClass('lte9')
+	    && !$('#app-content').hasClass('no-snapper')) {
 
 		// App sidebar on mobile
 		var snapper = new Snap({
@@ -1202,6 +1204,7 @@ function initCore() {
 			minDragDistance: 100
 		});
 
+
 		$('#app-content').prepend('<div id="app-navigation-toggle" class="icon-menu" style="display:none;" tabindex="0"></div>');
 
 		var toggleSnapperOnButton = function(){