Integrate push notifications

[ninsuo]

When users trigger people, they may miss late answers.
We may also integrate a chat sooner or later so executive people can discuss together around a trigger.

In order to notify people of new answer/new message etc, it will be a huge added value to integrate push notifications.

Product speaking:

• In homepage, invite to opt-in notifications
• In volunteer space, provide a way to to opt-out notifications
• Adapt GDPR data export/removal accordingly
• For all new answers coming >x mins after a trigger started, begin sending notifications

Useful links:
https://developer.mozilla.org/en-US/docs/Web/API/Notifications_API/Using_the_Notifications_API
https://developers.google.com/web/fundamentals/codelabs/push-notifications/
https://stackoverflow.com/a/33976929/731138