Canvis a "Push Notifications"

Ref. PWA13

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 registered participant in Decidim, I want to receive notifications even why I’ve left the website, for instance when there’s a new comment for a proposal that I’ve made.

Describe the solution you’d like

I want to receive these notifications on my device, for instance on my mobile phone, as other apps already do.

Describe alternatives you’ve considered

One possibility would be to not implement these notifications, but this doesn’t solve the main issue and also doesn’t help in the engaging area.

Additional context

Notifications examples from OneSignal.com

https://caniuse.com/push-api

Does this issue could impact on users private data?
It could have some personal data in some notifications (for instance, in the case of Messaging)

Related issues

At the technical level, this EPIC will have these related issues:

• Server setup (ref PWA13.1)
• Ask permission to the client (ref PWA13.2)
• Send notification to the client (ref PWA13.3)