Enhance styling of Qiita network views

[adswafford]

Flagging the group for input. We should make sure it looks nice before publication.

Here are some examples to consider:
http://visjs.org/examples/network/edgeStyles/colors.html
http://visjs.org/examples/network/edgeStyles/arrows.html
http://visjs.org/examples/network/nodeStyles/shapes.html

[josenavas]

Closing as the graph has been improved