Optimize web loading time (cache & main.dart.js)

[Petitoto]

Titan web loading time is still huge. We can improve it by:

• optimizing cache: we should use tags on files to allow both caching and web updates (now, the cache is disabled by nginx to prevent browsers caching old versions of Titan after updates)
• optimizing main.dart.js
  • splitting main.dart.js more by deferring loading of more classes
  • removing useless strings from main.dart.js: this still requires more investigations, but it seems that a lot of strings in many languages are uselessly stored in main.dart.js