Personalization

@Berdir asked specifically to review this branch, please leave the merging to him.
With the UserInterface typehint:

> Recoverable fatal error: Argument 3 passed to Drupal\simplenews\Form\SubscriptionsAccountForm::buildForm() must implement interface Drupal\user\UserInterface, string given in Drupal\simplenews\Form\SubscriptionsAccountForm->buildForm()

:confused: