YITH WooCommerce Affiliates Premium 3.15.0

New: support for WooCommerce 5.8
Update: YITH plugin framework
Dev: added new filter yith_wcaf_admin_tabs_control

* New: support for WordPress 5.7
* New: support for WooCommerce 5.1
* New: check over instance before processing payments
* Update YITH Plugin Framework
* Update: Italian language
* Tweak: get translated page of Terms and conditions with WPML
* Dev: added new yith_wcaf_set_ref_cookie filter
* Dev: added wpml config file to translate admin texts
* Dev: add a note to the payment when payment process doesn't belong to the correct instance

* New: support for WooCommerce 5.0
* New: German language
* Update: YITH Plugin Framework
* Update: Dutch language
* Fix: declared nonce to correctly save affiliate details
* Dev: added yith_wcaf_register_form action in registration form for logged users