Create GH Action for tearing down abandoned website preview environments

Occassionally, the tear down of Surge website preview environments fails for unknown reasons. Let's add a GH action, which is parameterized with a PR number and which is to be executed manually, and which tears down the given environment. This action should also log out the URLs of all the currently existing preview environments. See the release-contributors-list flow for an example of a GH Action of this kind.