EZP-22045: As a developer, I want user login to be fully handled by Symfony stack

https://jira.ez.no/browse/EZP-22045

Implements *login/logout* in Symfony security context.

[Documentation can be found here](https://github.com/ezsystems/ezpublish-kernel/blob/ac257859434c936ac0ca4259b6725a362109320e/doc/specifications/security/authentication_symfony.md).

Needs the following configuration in `ezpublish/config` files (see https://github.com/ezsystems/ezpublish-community/pull/87 ):

*security.yml*:
```yaml
security:
    firewalls:
        ezpublish_front:
            pattern: ^/
            anonymous: ~
            form_login: ~
            logout: ~
```

*routing.yml*:
```yaml
login:
    pattern:   /login
    defaults:  { _controller: ezpublish.security.controller:loginAction }
login_check:
    pattern:   /login_check
logout:
    pattern:   /logout
```

## Tasks

* [x] Implement base login/logout mechanism
* [x] Let security controller be more flexible (configurable layout/template)
* [x] Inject user in legacy
* [x] Remove legacy security listener
* [x] Ensure Symfony login is triggered when legacy sends 401 or 403 (`legacy_mode: false`)
* [x] Deactivate legacy `user/login` module if not in legacy_mode
* [x] Increase code coverage
* [x] Add documentation