Implement Push Notifications

[taoeffect]

Problem

The notifications in the app don't send push notifications.

Solution

1. Come up with a plan for push notifications, and post it to the comments below. Let us review and approve it.
2. Implement it. Use the PWA to test and verify that they work.
3. Send push notifications each time someone is @mentioend in a chatroom (or a DM is sent to them) while respecting their notification settings.
4. Send a single push notification when a new distribution period is starting (couple it with the existing in-app notification that's currently sent).

Suggestions and things to keep in mind:

• Because everything is e2e encrypted, the server won't know that someone was sent a message or a mention or whatever, and therefore the server can't be the one responsible for generating the push notifications in the first place
• Clients who are part of the group will need to be responsible for kicking off the push notifications, so for example, when I mention someone in the chat, my client will need to evaluate the other person's notification preferences, see if the user I mentioned accepts push notifications, and if so, call some backend API to generate the push notification

Useful projects:

• https://ntfy.sh/ (see hn discussion)
• https://github.com/caronc/apprise
• unified push => Unified Push
• Mastodon implements push notifications in its PWA fairly well

Research:

• Web Push notifications for Safari (blog)
• Web Push in Firefox
• Push on MDN
• Google (?) Push docs
• PWAs wont replace native iOS apps

Also, study the existing web-push library we're using for push notifications based on Sebin's PR.

Note

• If push notifications are being implemented as part of a PWA (instead of an app, or in addition to an app), issue PWA: create a progressive web app wrapper [$200 bounty] #926 needs to be closed first (creating a PWA out of this project).
• If push notifications are being implemented as part of an app bundle, then issue Investigate Electrino and Weex and Flutter and NodeGUI etc as Electron alternatives for Desktop #263 probably needs to be closed first.

Progress Update

The plan is to move Chelonia into the service worker, which will then receive and process push notifications in the background.

This was started in #1981, and then that was paused as we decided we didn't have time to finish this before release. Instead we made the necessary API re-arrangement in #2137 (which was merged) to support moving Chelonia into the service worker in the future. After it's moved into the service worker, then work on push notifications can begin.