Add Sigma.js functions to network display

[lehkost]

Desirable additions to how we use Sigma.js on the DraCor single-play pages:

• Zoom in/out button --> Add zoom in/zoom out buttons to the network canvas #137
• click on node shows other nodes directly connected to focal node (distance = 1) (see "Bind callbacks to events" example on Sigma.js homepage) --> Show connected nodes on click in network graph #258
• relate node size to word count of a character (using fixed numbers for min and max node size) --> Relate node size to word count of a character in network graphs #259
• relate node colour to gender information (male, female, unknown) --> Adjust sigma gender representation #184
• Search function for nodes (cf. here) (not for now)
• Colouring of clusters/communities (cf. here) (not now)

[cmil]

I think we should create separate issues for the items in the desirable additions list (where isn't one yet, and maybe even for the ones marked with "not now") and close this issue.

[cmil]

I created individual issues, closing this one.