Fix EZP-23033: DefaultPage setting injected from eZ5 into legacy, but it...

... can not be set via yml configuration

https://jira.ez.no/browse/EZP-23033

Makes it possible to define a `default_page` setting like it was in legacy.
Follow up : [Use this setting for redirection after login through Symfony](https://jira.ez.no/browse/EZP-23084) (should it be in?)