Hide Safari-Extension-Downloadlink while we don't have a working extension

[Jan Pfister]

the download link displayed on https://www.bibsonomy.org/gettingStarted when visiting with Safari currently links to an outdated and no longer working extension.

We should hide that download link (and possibly inform the user that our extensions in other browsers are still working?) until this is fixed: https://bitbucket.org/bibsonomy/bibsonomy-browser-buttons-extensions/issues/20/safari-extension-for-catalina-big-sur