YITH WooCommerce Affiliates Premium 3.15.0

* New: support for WordPress 5.4
* New: support for WooCommerce 4.0
* New: Greek translation
* New: added option to set up affiliates cookie via AJAX call (to better work with cache systems)
* New: added Elementor widgets
* Tweak: included variations in Excluded products field
* Tweak: include commissions and commissions history metaboxes into WC Subscription edit page
* Tweak: code reformat and improvements for PHPCS
* Update: plugin framework
* Fix: pending commission email for admin is not sent if the commission amount is zero or not exists
* Fix: removed duplicated id from form-referrer box
* Dev: added new filter yith_wcaf_customer_status_change_dashboard_url
* Dev: added new filter yith_wcaf_show_message_wc_print_notice

New: support for WooCommerce 3.9
Update: plugin framework
Update: Italian language
Update: Greek language
Update: Dutch language
Fix: system not recognizing correct value for "Pay only commission older than" option
Dev: added yith_wcaf_website_type filter

* Tweak: changed Fontello class names to avoid conflicts with themes
* Tweak: added checks before Fontello style inclusion, to load it just when needed