Details:
- Create another special page implementation that just displays the notice with a link
- Wire up that special page in place of the existing list properties page when federated properties is enabled
- The link to the source wiki will use the fed props URL configuration setting. pointing to ?title= and the specil page (see below)
The file with the wiring is repo/Wikibase.php, search for $wgSpecialPages['ListProperties']
URL construction would be something like this (taken from ApiEntityUrlLookup)
$this->sourceWikibaseUrl . 'index.php?' . http_build_query( [ 'title' => $titleText, ] );
