Web component to display entity graph

[gaetanmuck]

The goal here is to display relations between entities in a graphical way, that can be shown in a second tab of entity pages

The solution we thought of is using force simulation from d3.js.

Sources of inspiration:

• https://explore.gnd.network/gnd/118561448/relations?term=Johannes%20Kepler&rows=25&pos=1
• https://ontome.net/classes-tree
• https://ontome.net/class/61/namespace/187#graph

[gaetanmuck]

See issue #55