Polylang for WooCommerce 2.1

* Use composer for autoload
* Add debug information to Site Health
* Add compatibility with stock handling method to prevent race conditions when orders come in at the same time (requires WC 4.5).
* WooCommerce Product Bundles: fix cart translation

Fix payment link in customer invoice email

Add compatibility with Polylang Pro 2.7.4 (REST API for product categories and product tags)
Fix compatibility with WooCommerce Checkout Field Editor 1.3.6+
Fix a fatal error when WooCommerce Subscriptions deactivates itself
Fix a potential warning with WooCommerce Bookings