Push notifications

[jellybob]

Work in progress PR, this handles the absolute basics of registering for push notifications, and being able to send a notification to an endpoint.

Things still to do:

• Provide a reasonable UI
• Test on multiple platforms (so far only Safari on macOS has been tested)
• Job to send notifications on upcoming favourited content
• Job to notify of upcoming volunteer shifts
• Better handling of service worker updates
• Being able to open a specific page, and customisable title/body/actions
• Write some tests for send_queued_notifications and deliver_notification
• Handle replacement of notification endpoints

[wlcx]

Haven't looked through super closely, but looks great and could also fit really well alongside APNS push notifications for apple wallet tickets...

[jellybob]

This is probably in a reasonable state for testing now. No notifications are being queued other than those requested with the test button, but I think there's some value in wider testing of that basic functionality given how finicky WebPush has been for me so far.

You'll need to set WEBPUSH_PUBLIC_KEY and WEBPUSH_PRIVATE_KEY. You can generate a key pair with npx web-push generate-vapid-keys.

[jellybob]

I deeply dislike the approach being taken here, if anyone with sufficient SQLAlchemy knowledge wants to turn it into a single query with some joins I'd be very happy.