Implement Webhook system

[jefmoura]

Story

Webhooks are automated messages sent from an application, when something happens, to other ones. We want to implement a simple webhook system that knows only about Buildly Core models and when changes are made on one of them, information about it is sent to registered URLs. With this simple webhook, actions can be triggered in connected services because Buildly Core will notify the ones that are registered to the webhook system.

Tasks

1. Create a new webhook application Create a new webhook application #278
2. Create a list of available webhook events Create list of available webhook events #279
   3.Create API endpoints to register to webhook events Create API endpoints to register to webhook events #280
3. Integrate Celery into the project Integrate Celery into the project #281