Signals-based notification API and WebHooks support

by obvio171 for Review Board

ReviewBoard's increasing adoption across many diverse development environments has led to a need for integration with other tools that has outgrown its development capacity and focus, driving a growing need for extensibility. There is already an HTTP API and support for third-party extensions is on the way, but we still need a standard way to support event-based integration with other apps. This project aims to fill that gap by implementing support for WebHooks [1].