Implement trigger for opening Premium onboarding page

Background / User story

In #1444 (closed) we added an onboarding page for new Premium users. As part of that, we'd also like to open this page after users finished the Premium signup flow, for which we need to implement a way to trigger that from our website.

What to change

• Design: N/A
• Spec: spec!422 (merged) (only trigger for opening the Premium onboarding page)
• Development: None.

Hints for testers

• This change doesn't contain the Premium onboarding page (see #1444 (closed)).
• This change requires a change on the Premium web page. In the meantime, you can test it by adding the data-extension-page="premium-onboarding" attribute to the "Get started" button on the Premium web page.

Hints for translators

N/A