Web Push API support

[aeharding]

Requirements

• [X] Is this a feature request? For questions or discussions use https://lemmy.ml/c/lemmy_support
• [X] Did you check to see if this issue already exists?
• [X] Is this only a feature request? Do not put multiple feature requests in one issue.
• [X] Is this a backend issue? Use the lemmy-ui repo for UI / frontend issues.

Is your proposal related to a problem?

Users would like notifications for replies, mentions and DMs:

• without having the website foregrounded, and
• without installing a 3rd party native app.

Describe the solution you'd like.

Implement Web Push API

Describe alternatives you've considered.

No alternatives to the Push API exist.

Additional context

• iOS recently added support for Web Push API. Android has had support for a while. https://caniuse.com/?search=web%20push
• Mastodon supports web push natively, might be a good place to start!