fix zIndex overlay of notification over modal

app/assets/stylesheets/bootstrap.css

@@ -4806,7 +4806,6 @@ button.close {
   right: 0;
   bottom: 0;
   left: 0;
-  z-index: 1040;
   display: none;
   overflow: hidden;
   -webkit-overflow-scrolling: touch;

app/assets/stylesheets/zammad.css.scss

@@ -2359,9 +2359,9 @@ footer {
   max-width: 9999px;
   border-radius: 0;
   border: none;
-  z-index: 1110;
   color: hsl(206,7%,28%);
   box-shadow: 0 1px 14px rgba(0,8,14,.25);
+  @extend .zIndex-9;
 }
 
   .popover-body {
@@ -4141,6 +4141,7 @@ footer {
 
 .modal {
   position: fixed;
+  @extend .zIndex-10;
 }
 
 .modal-dialog {