Notifications: make them user friendly again

[manuroe]

There were a large amount of (legitimate) complaints on Goople Play and F-Droid flavours about how notifications are managed by the app since around version 0.7.5, ie, when we tried to manage android 8 restrictions enforcement on apps running in background.

The goal of this issue is to improve the UX providing more explanations to the end user because, on the codebase side, it is more a rollback in the time.

A lengthy story of the issue can be found at:
https://docs.google.com/document/d/1jy_Suwn7bFIRJA779nf6cKVlEkaOEsVTXvX9MOmYyMM/edit?usp=sharing.

This issue aims to:

• remove sticky notifications
• reintroduce the permission request to let the app run in background
• give explanations why the app needs it
• have a nice and explained fallback if the permission is not granted

Without breaking use cases listed at https://docs.google.com/spreadsheets/d/1ESuw6EUTjE9yOGNP9pCacuQdPLU1J70cvTl-TgWCHqc/edit?usp=sharing.

Here is the UX for Google play version: https://www.figma.com/proto/VKfJnrHSg5TFkGJTJUEk0rQW/Untitled?scaling=contain&node-id=82%3A566&redirected=1
No new UX on F-Droid version. Only no more sticky notifications like before.