Improved trust network visualisation

[thobson88]

We have a basic Graphviz network visualisation, but visually it could be much better. In particular, the DIDs should look more like those in the trustchain-mobile DID chain visualisation.

Eventually we should have an interactive visualisation, where a large network can be displayed nicely by making each node clickable, causing the view to zoom/focus on that node, with only its chain of upstream links and immediate downstream links visible.