Hide the link manager from the UI on upgrade, if the site has no links. New DB option, link_manager_enabled.
Enforce this by denying the 'manage_links' capability, which hides the All Links, Add New Link, and Link Categories screens. Hide WP_Widget_Links and the UI for the default_link_category as well.
Convert all references to 'posts and links' when handling reassignment on user deletion to just 'posts'.
see #21307.