queue messages into a channel that workers drain

[xavdid]

Per discussion in discord, refactor the notification service to queue up messages and immediately respond to incoming requests. This closely follows the structure of the Android version.

Considerations

• We'll now always send an HTTP 201 on notification, since we don't know yet if the push will succeed
• ❗ I did not run/test this code since I don't have an active Apple developer account and can't generate a certificate (that section of my UI is blank). The changes are simple enough, but please verify it still works before deploying.
• In the interest of simplicity, I didn't make the queue size and number of worker threads configurable. Happy to add flags for that (or change the defaults) if you anticipate wanting to tweak them.

[Gargron]

I think this has to be a continue, otherwise you're exiting the worker?

[xavdid]

Ah, you're totally right. That's a holdover for when I had the whole push in a goroutine. I can update it in by fork and send a new PR if you'd like, or you can just swap it.