WooCommerce Groups 2.8.0

* Added the groups_woocommerce_handle_woocommerce_subscription_status_updated filter.

* Replaced use of language domain constant with string literal.
* Updated compatibility with WooCommerce 3.3.
* Updated compatibility with WordPress 4.9.
* Updated access to product, documentation and support resources.
* Added the readme.txt.
* Fixed a rendering issue in the Groups product tab.
* Showing the right icon in the product tab.
* Updated the translation template.
* Updated the German translation.
* Updated the Spanish translation.
* Updated the French translation.

* Added support for subscriptions without parent order (WC >= 3.x only).