What push notification service should we use?

We need to integrate notifications for dwyl/smart-home-security-system#10. This issue is a summary of my research into different providers and techniques.

## Overview

All push notifications to mobile devices must go through either APNS (Apple Push Notification Service) or FCM (Firebase Cloud Messaging) depending in wether the device is running iOS *(APNS)* or Android *(FCM)*. These are the only services that each device natively listens to, so all other 3rd party services will pass messages through these at some point.

3rd party services will often offer extra features or have a different use case, e.g. Amazon SNS is a PubSub system with Mobile Push Notification support, not the other way round.

## Services

From some quick Googling etc, there seem to be a few options for push notifications

+ Firebase Cloud Messaging **(FCM)**
This is Google's messaging service, but can be used with other platforms as well

+ Amazon SNS
AWS' push notification service.

+ OneSignal
The first push notification service to come up on Google. Good SEO.

+ Twilio
Has been used before at Dwyl

Start by reading this provider comparison and the linked dwyl/smart-home-security-system#10 issue; compare APNS, FCM, Amazon SNS, OneSignal, and Twilio against the notification needs. Done means documenting a decided provider and the rationale for using it.

Written by the indexing model from the issue text.