Remove Twig form_themes

[Hanmac]

Feature Request

After sonata-project/form-extensions#444 gets into a Release,
then this can be removed:

SonataPageBundle/src/DependencyInjection/SonataPageExtension.php

Lines 35 to 45 in 489e2fa

	$bundles = $container->getParameter('kernel.bundles');
	\assert(\is_array($bundles));
	// add custom form widgets
	if ($container->hasExtension('twig')) {
	if (isset($bundles['SonataFormBundle'])) {
	$container->prependExtensionConfig('twig', [
	'form_themes' => ['@SonataForm/Form/datepicker.html.twig'],
	]);
	}
	}

This should be BC, and with a notice to remove the method and implements PrependExtensionInterface NEXT_MAJOR

[VincentLanglet]

sonata-project/form-extensions#444 is released @Hanmac
In 1.20.0 and soon in 2.0.0