Set subject on admin instance when extracting translatable strings

[jorrit]

Subject

After #6040 was added commands like php bin/console debug:translation execute methods on Admin class instances to extract translatable strings.

One of the methods is getForm, which calls configureFormFields. To tailor my forms I access $this->getSubject() in several of these methods. I hope this is not unusual or against best practices.

When the AdminExtractor calls getForm() no subject is present.

I added this code to make sure a basic model is available:

$admin->setSubject($admin->getNewInstance());

I am targeting this branch, because it is backwards compatible.

Closes #6221.

Changelog

### Fixed
- AdminExtractor sets subject created by `getNewInstance()` on Admin instance before extracting translatable strings.

[franmomu]

What about creating a simple Admin with that configuration and then check the catalogue? To reflect the reason why this is changed.

[jorrit]

I think it will take quite a bit of code to come up with a valid use case for having a subject in configureForm for instance. Do you have a suggestion?

[VincentLanglet]

You mean something like ?

protected function configureFormFields(FormMapper $form)
{
    $form->add('username');
    $form->ifTrue($this->getSubject()->isNewsletterSubscribed())->add('email')->ifEnd()
}

[jordisala1991]

Thank you @jorrit

[jorrit]

Thanks for merging. I hadn't actually added a test as suggested by @franmomu. I have tried, but it is quite hard in this test class to use a concrete admin class as the methods that are called by the extractor need a lot of dependencies.

[franmomu]

No problem, I'll try now with a functional test.

[jordisala1991]

I saw this test and I thought that was the requested one @jorrit . Should we revert?

[franmomu]

No, don't worry, I added one in #6237