SilverStripe CMS 4.0.0 issues a deprecation notice before calling
alternatePreviewLink on any page that hasMethod (i.e. is applied via an
extension such as SiteTreeSubsites). Instead of double-issuing a notice,
we will just mark this as deprecated in the next minor version via
docblocks.
A preview link must be loaded on the same domain the CMS is loaded
through, which was previously causing issues when a page (identified
via URLSegment) did not exist on the subsite domain. By _always_
prepending the identifier to the preview link, this should never happen.