Redoing how we fetch access tokens from WooCommerce Connect Server.

[rawdreeg]

Changes proposed in this Pull Request:

Must be tested together with https://github.com/Automattic/woocommerce-connect-server/pull/2563.

Refactors how oAuth access tokens are obtained. Instead of taking them from the incoming query string, a call to WooCommerce Connect Sever to exchange the incoming oAuth code for the access token is made.

Detailed test instructions:

1. Set up a local wcs environment using the woocommerce-connect-server's (https://github.com/Automattic/woocommerce-connect-server/pull/2563) branch.
2. Configure the extension to use your local WooCommerce Connect Server version by search-replacing api.woocommerce.com with your local wcs url through the includes/Handlers/Connection.php file of the plugin.
3. Try to connect your Facebook for WooCommerce plugin to Facebook.

Changelog entry

Update - Access Token retrieval procedure.

[message-dimke]

I did test the update and it works. The only minor thing is I had to use wp_remote_get instead of the safe version of it when testing locally. Must be related to some certificate checks.

LGTM! Thank you, Rodrigue!