This allows admins to configure whether contact site support is
available to everyone, authenticated users, or nobody.
The behat testing checks linked and direct access for each setting,
as well as adding testing that the support page override works as
expected.
Removes $PAGE->set_secondary_navigation(false) calls which certain pages
were using to prevent displaying secondary navigation menu in the system
context. This logic is now generally handled within the secondary
navigation class.
