Extract admin strings

Adds app.trans calls for strings used by the admin UI.
- Strings for AddExtensionModal.js not included.
- Corresponding YAML will be sent later w/ more extracted strings.

js/admin/src/components/PermissionsPage.js

@@ -25,7 +25,7 @@ export default class PermissionsPage extends Component {
               ))}
             <button className="Button Group Group--add" onclick={() => app.modal.show(new EditGroupModal())}>
               {icon('plus', {className: 'Group-icon'})}
-              <span className="Group-name">New Group</span>
+              <span className="Group-name">{app.trans('core.admin.permissions_new_group_button')}</span>
             </button>
           </div>
         </div>