Fixed a bug where an entity would be related to a page, but not appear in the associator as something to select / deselect.
The issue was that items were not appearing as associated with a page if they were no longer shared by the owner.
The behavior now is that an item borrowed by a site will always show up in the associator interface. If the owner of an item changes the status of an item to "private," this means it can not be borrowed in the future - it no longer deletes existing site_borrows_entity relationships.
The sharing interface will now place a notice next to items that will not be available for borrowing once unborrowed.