Translate preferences page to the new order.

Translate preferences page to the new order.

Also, this change fixes much of defect 18 ("cookie expiry causes user
to be silently logged out and not subsequently redirected to login
page").  This is implemented by giving the actions table a list of
rights suitable for the action at hand and redirecting to the login
page on failure.  Even better would be to redirect back to the
problematic page on success.

Unusually for this branch there is a server change here, implementing
the long-standing TODO that the server should remove preferences if
you set them to their default value.  This removes this logic from the
web interface.

It remains the case that the web interface has to work out some of the
defaults.  However, that's left for another day.