Add the missing libresign/saas-onboarding pattern #19

libresign_theme_prepend_saas_onboarding_to_content() in inc/account.php prepends <!-- wp:pattern {"slug":"libresign/saas-onboarding"} /--> to the shop, checkout and order received pages, but no pattern with that slug is registered, so nothing is rendered there.

Either the pattern needs to be created, or the prepend removed if the onboarding is no longer wanted. In both cases, an integration test should cover the shop, checkout and order received pages.

Raised in #14 (comment).