Fixes clicking on the group-cell to change settings

Now, the user can click on the whole group-cell to change some settings like groups, gender and member-number.

client/src/app/site/users/components/user-list/user-list.component.scss

@@ -18,6 +18,7 @@
     }
 
     .mat-column-group {
+        z-index: 3;
         flex: 2 0 60px;
     }