Description:

This PR fixes this error:

Uncaught PHP Exception Symfony\Component\Debug\Exception\FatalThrowableError: "Symfony\Bundle\FrameworkBundle\Controller\Controller::redirect(): Argument #1 ($url) must be of type string, null given, called inapp/bundles/PageBundle/Controller/PublicController.php on line 64" at vendor/symfony/framework-bundle/Controller/ControllerTrait.php line 104

It uncovered that Landing Pages with configured redirect can be saved in an inconsistent state without the URL to redirect to. This PR fixes this to redirect to the 404-not found page instead instead of throwing the 500 error. And for the new pages it adds a validation to stop users creating such landing pages.

Plus:

1. It simplifies the JS by using Mautic's system to show/hide fields based on another field value.
2. It simplifies the code as it removes the unused constructor and config param that should have been removed in https://github.com/acquia/mc-cs/pull/553
3. If an invalid URL was added to the redirect URL field, it prepended http:// to it and made it a valid URL. This behavior was removed and instead the validation message will be shown.

Steps to test this PR:

1. Open this PR on Gitpod or pull down for testing locally (see docs on testing PRs here)
2. Create a new landing page
3. Set the Redirect and do not set the redirect URL (it will not allow you to save such state with this PR)
4. Unpublish the page
5. Save
6. Test the page in an incognito browser window. Even if you have a page with the redirect but not redirect URL either from before this fix was applied or via direct database manipulation, it will not error out but show the 404 page instead.

Other areas of Mautic that may be affected by the change:

1. Just landing page form and API

List deprecations along with the new alternative:

1. None

List of areas covered by the unit and/or functional tests:

1. The tests are covering the existing pages in this inconsistent state that they will redirect to the 404 page
2. And that the proper validation message will be shown for empty and invalid URL values.
3. It also tests that the page will be saved with the valid URL.