Fix EZP-22099: Support of legacy sso handlers to continue to work with new authentication system

https://jira.ez.no/browse/EZP-22099

This PR brings support for [legacy SSO handlers](http://share.ez.no/learn/ez-publish/using-a-sso-in-ez-publish).

To be able to use your legacy SSO handlers, use the following config in your `ezpublish/config/security.yml`:

```yaml
security:
    firewalls:
        ezpublish_front:
            pattern: ^/
            anonymous: ~
            # Adding the following entry will activate the use of old SSO handlers.
            ezpublish_legacy_sso: ~
```

## TODO
* [x] Add support for legacy SSO handlers.