Add third tier to key namespacing

- Changes all `app.trans` calls to `app.translator.trans` calls.
- Changes existing keys to [three-tier namespace structure](https://github.com/flarum/english/pull/12).
- Extracts additional strings for `lib:` namespace.
- Extracts two previously missed strings for EditGroupModal.js.

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">{app.trans('core.admin.permissions_new_group_button')}</span>
+              <span className="Group-name">{app.translator.trans('core.admin.permissions.new_group_button')}</span>
             </button>
           </div>
         </div>