Browser-level notifications

[drusepth]

Reference:

It'd be nice to have browser-level notifications for things like thredded notifications or site notifications, that can be turned on from a page in settings that lets you "subscribe" to whatever updates you want (and all be off by default).

Depending on how browser notifications work (webhooks 🤞 , this seems like it would be reasonably easy to trigger in a worker spawned from new Thredded::Post, etc.