* refactor and reorder checkout processing
* move experimental order_processed hook as last step before payment:
+ flesh out docs - intention/purpose, introduced version, link to PR,
relationship to legacy hook
* return current user_id with checkout error responses, so UI can update
* clarify user id comment
* Add store API nonce to all responses
* catch new nonce & update store for POST /checkout errors
* ensure $order hook param and database are in sync
* fetchResponse->errorResponse
* Move payment method ID validation to schema
* Add the nonce inline
* Simplify error method
* Add todo for injection
* Hook docs
* Return user ID as a header instead of in error response
* Default to private methods
* Tidied processing logic/order save placement
* White space removal
* Remove duplicate comment
* Remove root slash
* Remove unused created variable
* Remove woocommerce_registration_error_email_exists filter in error message from API
* Move payment validation
* Moved comments
Co-authored-by: Mike Jolley <mike.jolley@me.com>
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