Enable payment methods to display a success message

Not all themes provide a .form-actions wrapper so that might not be
reliable.

This makes it available to use it to place the success message.

Debug "called method validate() on NULL" error

Pass form data to AJAX callback

[fix] Fix assignment for the form callback

Using a += assignment here made it impossible to override some of the
method element’s properties (eg. #attached).

We still have to support returning an otherwise empty form-API array as
some payment methods rely on webform_paymethod_select adding its wrapper
properties back.

CI: Test with PHP 7.4

The log messages should be visible as breadcrumbs in sentry.

The function gets the $element by value anyway. Also using references
implicitly creates a NULL value if the variable doesn’t exist which
hides possible errors.

Results: Remove support for the patch from #2117285.

Implement webform results in a way that works with any patch version.

- Unpatched webform.
- With old versions of #2117285 (up to and including comment 49).
- With newer version (based on #3086038.

Remove workaround for async loading of JS.

Improve form_state handling in AJAX callbacks.

We don’t have `$form_state['storage']` on the first page of a webform.

- `$form_state['values']` is always empty because the values are not
  stored in the form cache.
- Use `$form_state['page_num']` to verify we are on the same page. If
  not return a HTTP 400.
- Use `$form_state['storage']['submitted']` to construct the submission.

Save the submission when the payment is saved in AJAX callbacks

Add hook for altering the payment