Fix EZP-23074: Missing form token meta tags when browsing legacy modules

https://jira.ez.no/browse/EZP-23074

Alternative of https://github.com/ezsystems/ezpublish-legacy/pull/1026

This acts with a Response listener, triggering `ezxFormToken` output filtering once again, to ensure meta tags containing CSRF token are present.

Note that `ezxFormToken::output()` has been slightly modified as well (see https://github.com/ezsystems/ezpublish-legacy/pull/1029).

## About cache
There is nothing to worry about here since HTTP cache is assumed to be handled by legacy modules.
**`LegacyKernelController` does not set cache headers on its own**.

See [`LegacyResponseManager::mapHeaders()`](https://github.com/ezsystems/ezpublish-kernel/blob/master/eZ%2FBundle%2FEzPublishLegacyBundle%2FLegacyResponse%2FLegacyResponseManager.php#L141) for more insight.
Instead of a comment, use a private. Suggestion:
```
if ( $this->isLegacyResponse( $response ) )
```