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

*DO NOT MERGE*

Implements *login/logout* in Symfony security context.
Needs the following configuration in `ezpublish/config` files:

*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
* [ ] Let security controller be more flexible (configurable layout/template)
* [ ] Inject user in legacy
* [ ] Increase code coverage
* [ ] Add documentation