[FIX] web: kanban dropdown-menus were not working in mobile mode

In mobile mode, dropdown-menus are working with a dropdown backdrop for
mobile compatibility. As the z-index of kanban dropdown-menus were now
forced to 1 for no good reason, those appeared under the backdrop and
were thus not working.

addons/web/static/src/less/kanban_view.less

@@ -88,13 +88,12 @@
     position: relative;
     background: white;
     border-color: @gray-lighter-darker;
-    z-index: 2; // Make the dropdown link appear over the dropdown-menu
+    z-index: @zindex-dropdown + 1;
 }
 .o-kanban-dropdown-menu() {
     .o-position-absolute(@right: -1px);
     margin-top: -1px;
     border-color: @gray-lighter-darker;
-    z-index: 1; // Make the dropdown-menu appear under the dropdown link
 }
 .o-kanban-colorpicker() {
     max-width: 150px;