Restore the userCreated event

[wyattjoh]

What does this PR do?

• Re-introduces an event when a user is created (see below): fixes Event service no longer exists, not sure how to trigger on events now #1426
• Modifies the createLocalUser and findOrCreateExternalUser functions to accept a context object as the first parameter that it can use to fire the event.
• Introduces a graph subscription for users that are created.

An example of utilizing the new event is as follows:

module.exports = {
  connect(connectors) {
    const { graph: { subscriptions: { getBroker } } } = connectors;
    
    // Get the handle for the broker.
    const broker = getBroker();

    // The passed in method will fire for every user that is created.
    broker.on('userCreated', user => {
      // Do something with the user...
    });
  },
};

You can also listen with a graph subscription to userCreated.

Note: External plugins utilizing the Users service's user create methods must adjust their call signatures to pass in the context.

[alinhle]

LGTM