Fix missing function error in studip action menu, fixes #807 (#808)

Co-authored-by: Dennis Benz <dennis.benz@uos.de>

vueapp/components/Studip/StudipActionMenu.vue

@@ -118,7 +118,7 @@ export default {
             });
         },
         shouldCollapse () {
-            const collapseAt = this.collapseAt ?? this.getStudipConfig('ACTIONMENU_THRESHOLD');
+            const collapseAt = this.collapseAt ?? window.STUDIP.config['ACTIONMENU_THRESHOLD'];
 
             if (collapseAt === false) {
                 return false;