Notifications System Core

[ChrisPenner]

Overview

TODO:

• Add checks to ensure the subscriber has access to subscribe to notifications on a given resource/thing Changed it to do a check before creating the notification rather than before creating subscription.
• Record notification events on contribution create and branch update events
• Add bulk notification status event update APIs.

Adds the core tables and APIs for managing the notifications system.

MVP supports only a small subset of what's to come.

MVP:

• allows subscribing to contribution creation events within a user or project
• allows subscribing to branch update events within a user or project
• provides APIs to back a Notifications Hub
• includes the backbone for email and webhook notifications, but they're not wired in yet.