Public Mailing View page doesn't render hook tokens

Generic hook tokens are inaccessible from the mailing view page when the contact is not logged in. However, some tokens (e.g our token to render a drupal view of civimail or drupal content within the body of a newsletter) don't render in view mode.

I have & will submit a PR for a patch that fixes this. However, it works by calling the token hooks with contact_id = 0. This could upset some custom hooks - e.g if they were calling civicrm_api3('contact', 'getvalue', array('contact_id' => $contactID) without a try /catch block.