Redirect to event instead of registration detail page

[JPustkuchen]

After successfully submitting the registration form the user is being redirected to the detail form of the registration (bad UX: without a link back to the source event), or to the website front page if he has no permission for registration access.

It's controlled by:

    if ($registration->access('view')) {
      $form_state->setRedirectUrl($registration->toUrl());
    }
    else {
      $form_state->setRedirect('<front>');
    }

in Drupal\rng\Form\RegistrationForm

This doesn't seem the best option for most cases. I'd suggest to add a setting to select whether the user:

Is redirected back to the event entity
Is redirected to the registration detail page + add a link to the parent event entity
(3. Is redirected to a custom (token) path)

What do you think?

[JPustkuchen]

I created a pull request for a first simple but helpful change to redirect to the event instead of the frontpage. That doesn't require huge changes and is a first good improvement. See #161