Fix EZP-21924: [SensioLabsInsight] Refactor LegacyKernelController to be less than 20 lines

https://jira.ez.no/browse/EZP-21924

See https://insight.sensiolabs.com/projects/0885c0ce-4b9f-4b89-aa9c-e8f9f7a315e0/analyses/7#rule-24-034

This PR refactors `LegacyKernelController` to be thiner and delegates `LegacyResponse` generation to new `LegacyResponseManager` service.

Note that `mapHeaders()` cannot be tested because of [issues in PHPUnit with process isolation](https://github.com/sebastianbergmann/phpunit/issues/314) (see also [this issue on testing headers with PHPUnit](https://github.com/sebastianbergmann/phpunit/issues/720))