Canvis a "Custom offline fallback page"

Ref. PWA10

NOTE: this feature is from a big development. See PWA discussion at GitHub.

Is your feature request related to a problem? Please describe.

As a visitor, when I don’t have connectivity and enter a Decidim page what I get is the “No internet connection” screen (aka Downasaur).

Describe the solution you’d like
I would want to see a personalized error message. For doing this exists the ServiceWorker API. It could also serve us as a building block for having other kinds of features with better offline support, such as being able to…

• … read my messages
• … send new messages when I get connectivity again
• … download some parts of the platform (like all the contents of a given space)

Describe alternatives you’ve considered
To use something like Cordova or Ionic, but would be much more expensive to implement given our current architecture.

Additional context

Lighthouse relevant section

Does this issue could impact on users private data?
Could be, as this could potentially enable us to download some private data to the participant's mobile phone or browser, and an evil actor with local access to her device could read that data. At the moment we will only show a friendlier nonconnected page.

Acceptance criteria

• Given that I’m a visitor that has already visited once Decidim and I’m not online,
  When I go again
  Then I see a custom error message.