Add listeners for CustomEvents to handle built-in UserKit events.

Performing an action when the UserKitWidget has finished initialization:

document.addEventListener("UserKitInit", function() {
	// the UserKit widget has finished initialization

Performing an action when a user has signed in

document.addEventListener("UserKitSignIn", function() {
	// user has signed in by either:
  // a) resuming a previous session on page-load
  // b) a successful login through the login flow

Performing an action when an invite is accepted.

document.addEventListener("UserKitInviteAccepted", function(event) {
	// user has accepted an invite
  // `event.detail` will contain the accepted invite-token