Add basic support for webhook registration and handling

[jmurty]

Add basic support for registration and handling of Shopify webhook notifications, as a starter example.

Key pieces:

• registerWebhook function in lib/shopify.js registers a webhook via the admin API; creates a new registration or updates an existing one as appropriate
• add endpoint /api/shopify/webhook/TOPIC/ that receives, verifies, and prints to console the content of webhook notifications
• register the app/uninstalled webhook after initial auth, like the original example Shopify project

Also includes an endpoint /api/shopify/register-webhooks you can hit manually from within the Shopify Admin 1 to register webhooks, as a convenience to add or update webhooks since the after-auth hook is rarely triggered (unlike in the original Shopify example)

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