Remove use of global \Drupal::configFactory() call from forms

In RecurlySubscriptionPlansForms the submitForm method uses \Drupal::configFactory(). The use of \Drupal in generally frowned upon from my understanding, so the config.factory service should be included with dependency injection. However, in this case it already is available to us through the FormBase class that we are extending.

So...

\Drupal::configFactory()->getEditable('recurly.settings')->set('recurly_subscription_plans', $recurly_subscription_plans)->save();

... becomes...

$this->config('recurly.settings')->set('recurly_subscription_plans', $recurly_subscription_plans)->save();

This might be in other spots throughout the codebase too, but hopefully this sheds some light on the issue so it can be fixed everywhere!