Fix: sonata_user_already_authenticated was not being translated (#1265)

[BitScout]

Subject

Fixes the problem of the message "sonata_user_already_authenticated" not being translated.
The message is triggered when going to the login page while logged in.

I am targeting this branch, because it is the current version and this is where the bug was found.

Closes #1265.

Changelog

### Fixed
- Fixed the problem of the message "sonata_user_already_authenticated" not being translated.

[franmomu]

And can you please add a NEXT_MAJOR comment above the constructor like this:

// NEXT_MAJOR: Make $translator argument mandatory.
public function __construct(

So we don't miss it in 5.x.

[VincentLanglet]

Thanks