YITH WooCommerce Stripe Premium 3.20.0

* Tweak: removed paymentIntent creation on is_available method, to avoid excessive API calls & Webhooks requests (thanks to Jeremy & Scott)
* Fix: error on session creation when user is guest (thanks to Paul & Maurice)

* New: WooCommerce 3.7.0 RC2 support
* Tweak: added check over gateway instance before calling subscription methods
* Tweak: added default card template, when token is not a Stripe one
* Tweak: use .length() instead of .size(), to prevent problem with new jQuery version
* Tweak: add a function_exists to prevent problems with a previous version
* Tweak: add maxlength to Expiration date
* Update: internal plugin framework
* Update: Italian language
* Fix: calculate cart totals before sending it to stripe, to make stripe checkout work with gift card
* Dev: added method to API class, to update cards

* New: Support to WordPress 5.2
* Update: Plugin Framework
* Tweak: get_home_url() to get_site_url()
* Tweak: remove protocol from url before performing security check
* Dev: Added new filter yith_wcstripe_charge_params