Fix uninitialized is_admin value.

If trying to access an application, using the secret URL, as that user,
we try to redirect that user to the non-secret URL.

However, this redirection was not working, because get_url tries to
access auth['is_admin'] which was not initialized.

Change-Id: Ib8a5e25da0af56aad4096f9e3e292a02e18bc4a1