Plans in Site Creation: Integrate Plan selection into Site Creation flow

[staskus]

Description

Integrate Plan selection into Site Creation flow

Requirements:

1. Enable flow: Theme Selection -> Domain Selection -> Plan Selection -> Site Assembly -> Checkout
2. Use the Plan selection view developed on Calypso Task: Create Plans page for Jetpack App Site Creation Automattic/wp-calypso#82135 and display on a web view: wordpress.com/jetpack-app-plans/
3. If paid domain is selected, pass paid_domain_name parameter. Example: paid_domain_name=domainname.com
4. If paid plan is selected, the original URL will redirect with plan_id and plan_slug parameters. Example: wordpress.com/jetpack-app/plans?plan_id=1009&plan_slug=personal_bundle
5. Add the selected paid plan to the cart (Plans in Site Creation: Create functionality to add domain with a plan to a cart WordPress-iOS#21636) using plan_id
6. If free plan is selected, the original URL will redirect with plan_slug=free_plan parameter without a plan_id.Example: wordpress.com/jetpack-app/plans?plan_slug=free_plan
7. Don't include paid domain into the cart if free plan is selected
8. If paid plan + domain is added to the cart, present Checkout after Site Assembly

[staskus]

Use Calypso Live URL (Automattic/wp-calypso#82404 (comment)) instead of wordpress.com/jetpack-app-plans while Automattic/wp-calypso#82135 is in review