Add tests that notification preferences are respected;
also move existing notification tests to the new test case
Add extra UserPagePermissionsProxy methods:
    - can_edit_pages
    - publishable_pages
    - can_publish_pages
Add tests for new UserPagePermissionsProxy methods
Filter notification preferences form fields according to user permissions
Add UserPagePermissionsProxy tests for a user who can access the admin only
Only show the link to notification preferences if the user has preferences to set
Add tests for notification preferences views' being filtered for less privileged users