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

[Marc-Andrieu]

Partly closed by the #653-#659-#663 trio on web optimization.
Update 2.5 years after:

• cache: I think we really can cache assets, fonts, icons, CSS, etc. (Obviously the best thing would be to include the Titan version in filenames so that a new version makes the browser fetch new files bc it can't fetch unseen filenames from its cache)
• deferring (for JS only): every single router now defers every single page... Idk if we can defer even more in a maintainable way to load only what's necessary (defer the loading of large libraries, using show, defer widgets or providers...)
• pseudo-dead code main.dart.js: interesting, I've never tried to read the content of this huge JS file.