Push notifications for mobile

## Problem

Currently when Convos is in the background on Android notifications don't appear which means the only way to know if you have notifications is to open Convos. As Convos can be installed as a PWA through Chrome or Firefox on Android, I think not having push notifications should be considered as missing an essential feature.

## Solution

The best solution is to use the web push API^{[[1]](https://developer.mozilla.org/en-US/docs/Web/API/Push_API)[[2]](https://developers.google.com/web/fundamentals/push-notifications/how-push-works)} as it doesn't require setting up FCM (this is handled underneath by the browser^{[[3]](https://developers.google.com/web/fundamentals/push-notifications/faq#what_is_the_deal_with_gcm_fcm_web_push_and_chrome)[[4]](https://developers.google.com/web/updates/2016/07/web-push-interop-wins)}, which itself uses FCM on Chrome or Autopush for Firefox and thus doesn't effect battery life), is an open standard and which creates native notifications on Android. This is currently how The Lounge has push notifications implemented^{[[5]](https://github.com/thelounge/thelounge/pull/1124)}.

## Previous Issues

https://github.com/convos-chat/convos/issues/312, https://github.com/convos-chat/convos/issues/493.

[1] https://developer.mozilla.org/en-US/docs/Web/API/Push_API
[2] https://developers.google.com/web/fundamentals/push-notifications/how-push-works
[3] https://developers.google.com/web/fundamentals/push-notifications/faq#what_is_the_deal_with_gcm_fcm_web_push_and_chrome
[4] https://developers.google.com/web/updates/2016/07/web-push-interop-wins
[5] https://github.com/thelounge/thelounge/pull/1124

Review previous issues #312 and #493, then compare the proposed Web Push API approach with The Lounge pull request #1124 and the linked API documentation. Done means Convos delivers native Android notifications while running as a PWA in the background.

Written by the indexing model from the issue text.