Add coupon description to REST API coupon response
Add shop_webhook custom post type

Part of #5564
Add webhooks REST API endpoint

Part of #5564
Introduce WC_Webhook class

Part of #5564
Hide webhook comments

Part of #5564
Add shop_webhook capabilities on install/upgrade

Part of #5564
Load webhooks on each page load

Part of #5564
Merge pull request #5941 from maxrice/feature-issue-5564-rest-api-webhooks

Webhooks Implementation