Implement web push notifications

[punchagan]

I think it would be useful to add push notifications to the app. Everything that currently has Zulip notifications could become web push notifications - new events, new posts, new pictures.

This could potentially remove the need for an event/post creator to share the link to it on the group. There will be some people who don't want to have notifications turned on, but the expectation would be for them to check the app themselves. Or we could share the link in the group, if needed - like not enough call-ins, etc.

Some resources for implementing this:

• https://developers.google.com/web/ilt/pwa/introduction-to-push-notifications
• https://code.luasoftware.com/tutorials/pwa/develop-web-push-notification-server-with-python/
• https://code.luasoftware.com/tutorials/pwa/setup-push-notification-for-pwa/
• https://medium.com/commencis/web-push-notifications-with-service-workers-cf6ec8005a6c
• https://medium.com/izettle-engineering/beginners-guide-to-web-push-notifications-using-service-workers-cb3474a17679

[punchagan]

This could be useful for letting people RSVP. If we can provide yes, no buttons that would be great. (Idea - Birdie)