fullcalendar.scss: display header as rows in mobile view

resources/assets/stylesheets/fullcalendar.scss

@@ -236,9 +236,19 @@ a.fc-event, td.fc-event {
 // Responsive
 .responsive-display {
     .fc-header-toolbar {
+        flex-direction: column;
+
+        .fc-left, .fc-center, .fc-right {
+            width: 100%;
+            text-align: center;
+        }
+        .fc-left, .fc-center {
+            margin-bottom: $line-height-computed - $font-size-base;
+        }
+
         .fc-button-group {
             .fc-button {
-                min-width: 0;
+
             }
         }
     }