Theme Purchase: After Checkout, Send to Congrats Page

[Copons]

If a theme requires a purchase (a plan upgrade and/or a subscription), after the Checkout and Purchase, we should send the user to the Congrats page where they can activate the theme.

Note: the theme must not be automatically activated behind the scenes, but it must be activated by the user (aka 2-step theme activation).

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The Congrats page: