Activation: Decouple WooCommerce Payments setup from WooCommerce Admin

[becdetat]

Move Payments setup logic from WooCommerce Admin in to WooCommerce Payments and use addFilter/applyFilter to decouple it from WooCommerce Admin.

Note that there is a related ticket in #1477 which requires a 50/50 A/B test on the existing flow vs this new stepper flow. #1477 should probably be done in parallel with this ticket as the existing flow will need to be retained.

Tracking:
wcadmin_wcpay_setup_stepper_view - when the user views the stepper flow
Event prop: wpcom_connected : Y/N - identify if the user is has a WordPress.com account

Check the parent issue for any updates that might have been missed in this issue.

[mattsherman]

Spike from @becdetat using filters to decouple: woocommerce/woocommerce-admin#6755

[mattsherman]

I'm going to spike a slot-fill approach.