Rev. 2f7c03d added a second admin user (id 2), the 'human' one, on
top of the 'technical' one (id 1, also known as the superuser).
Basically, all calls to _is_superuser() should have been changed to
_is_admin(). They weren't. As a consequence, some features that
were previously available for the admin weren't anymore (e.g.
tours).
This rev. replaces all calls to _is_superuser() by calls to
_is_admin().