Assistant: Hide all navigation buttons on the last page

When no goa accounts are present, we would forget to hide the skip
button, leading to badness.

https://bugzilla.gnome.org/show_bug.cgi?id=737130
assistant: Work around a GTK+ problem

When all the buttons get hidden, there are visible artifacts on
the last page (below the place where the buttons used to be), unless
we force a resize of the headerbar.