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
* https://jira.ez.no/browse/EZP-23084

Makes it possible to define a `default_page` setting like it was in legacy.
This PR *fixes* [EZP-23094](https://jira.ez.no/browse/EZP-23084), so that this new setting is used for redirection after login through Symfony, overriding `default_target_path` from Symfony SecurityBundle.
Note that, if present, the target path in the form will always have precedence (Symfony behavior is not changed). See [Symfony doc on redirection after login](http://symfony.com/doc/2.3/reference/configuration/security.html#redirecting-after-login).

## Usage
Default page can easily be configured by SiteAccess:

```yaml
ezpublish:
    system:
        ezdemo_site:
            default_page: "/Getting-Started"

        ezdemo_site_admin:
            # For admin, redirect to dashboard after login.
            default_page: "/content/dashboard"
```

## Order of precedence
This setting **does not change anything to Symfony behavior** regarding redirection after login. If set, it will only substitute the value set for `default_target_path`. It is therefore still possible to specify a custom target path using a dedicated form parameter.

**Order of precedence is not modified.**