Retain transaction ID through 401 pages
    
    If we get a 401 during a transaction (i.e. failing authentication),
    ipsilon shows the 401 error page, which has a link back to the login
    page.  Clicking this link causes a new transaction to be created,
    losing any in-progress service provider authentication, and not
    redirecting back to the SP after a successful transaction.
    
    If we hit the 401 page and there's a current transaction, add the
    transaction ID to the link back to the login page, so the current
    transaction is preserved.
    
    Merges: #121
    Signed-off-by: Howard Johnson <merlin@merlinthp.org>
    Reviewed-by: Patrick Uiterwijk <puiterwijk@redhat.com>