Allow gateways to specify redirect URL

Part of #3396
Move /add-payment-method/ endpoint to /my-account/

Part of #3396
Ukrainian language
Use new ID for add pay method form

Part of #3396
Add new notices API for blue info message

Part of #4034
Use wc_add_notice() when updating shipping costs

Fixes #4034
Use new notice API for "Add a Pay Method" notice

Part of #3396
Add get_woocommerce_api_url() method
Remove WP-API library
Add REST API request/response interface
Add REST API JSON request/response handler
Add REST API XML request/response handler stub
Add new WC_API_Server class

Based on WP_JSON_Server, this class add response handling based on
endpoint suffixes (.json or .xml) or the ACCEPT header. It also removes
some unneeded functionality and renames filters so as to not conflict
with users who may have WP-API installed already. Finally, instead of
no authentication required by default, authentication is always
required unless specifically disabled via filters.
Update API classes to use new WC_API_Server class
Update core WC_API class to use new WC_API_Server class
Define and use WC_API::VERSION constant for endpoint versioning
Merge pull request #4060 from AndriyGusak/master

Ukrainian language
payment heading
removing extra slashes when order meta is saved in admin
Set correct version on notices.php template