Launch: add Calypso "site launched" event

[simison]

Add new calypso_site_launch event at the point when site is launched via all flows (starting in editor, in customer home or in settings).

The event should these props:

• flow: slug
• site title: true|false or the actual string (TBD)
• plan: slug
• paid domain: true|false
• theme: slug

[github-actions]

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