[FIX] web_kanban: more strict position for dropdown

Wherever the dropdown is in the dom, it should always be displayed
at the kanban card top right corner.

addons/web_kanban/static/src/less/kanban_view.less

@@ -65,7 +65,9 @@
         }
 
         .o_dropdown_kanban {
-            float: right;
+            position: absolute;
+            top: 5px;
+            right: 10px;
             @media (max-width: @screen-xs-max) {
                 visibility: visible;
             }