Don't restart force layout on resize.

Choose arbitrary fixed dimensions (200 x 200) for the force layout,
then use SVG viewBox to keep the fixed range centered.
This means that we don't need to tell the D3 force layout when the
visualisation changes size. It avoids the distracting drift when,
for example, opening and closing the side tabs. Also makes the position
of fixed nodes more consistent with the centering gravity force.