Improve perceived performance

[cmil]

Since we depend on several bulkier Javascript libraries (yasgui, swagger-ui-react, sigma among others) the javascript and css code to be loaded initially has grown quite big. Even with code splitting it takes a while to load the dracor.org frontend for the first time. Here are a few things we could do to improve the perceived performance of the site:

• add loading indicator
• refine code splitting (e.g. for sigma)
• reduce dependencies (e.g. JSNetworkX)

[cmil]

JSNetworkX is gone now (see #59).