Sulu: Fix authorization bypass when creating preview links

Sulu is an open-source PHP content management system based on the Symfony framework. Prior to versions 2.6.25 and 3.0.8, the preview-link endpoint and src/Sulu/Bundle/PreviewBundle/Application/Manager/PreviewLinkManager.php do not enforce VIEW permission for the target resource in PreviewLinkManager::generate() or PreviewLinkManager::revoke(). An authenticated administration user who knows a target resource identifier can create or revoke a preview link for any page, article, or snippet, including content in a webspace or area the user cannot view. A generated preview URL is public and resolves content by an opaque token, allowing the user or anyone receiving the link to read restricted content without authentication. This issue is fixed in versions 2.6.25 and 3.0.8.