Hooks helper

[perry-mitchell]

Add a hooks helper for supporting webhooks. Hooks should be able to be added to the collection and fired with the relevant data when a condition is met.

Hooks should be stored in the data store like scheduled tasks, for example.

[perry-mitchell]

Some idea for hooks:

• A job succeeds
• A job fails
• A job is added
• A job starts
• A job stops

The job-related hooks should use the following structure (JSON payload):

{
	"type": "job-started",
	"time": 1554975865811,
	"job": {
		"id": "..."
	}
}