Permission Grid: stick the headers to handle a lot of tags (#2887)

* sticky thead th & tbody th, adapt PermissionPage and PermissionGrid layout

* adjust height size

* cleanup

* cleanup with some SychO9's recommendation

* remove the `thead th` `first-child` "protection"

js/src/admin/components/PermissionGrid.js

@@ -17,7 +17,7 @@ export default class PermissionGrid extends Component {
       <table className="PermissionGrid">
         <thead>
           <tr>
-            <td></td>
+            <th></th>
             {scopes.map((scope) => (
               <th>
                 {scope.label}{' '}