Upgrade success page headings need more spacing

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### Problem to solve

At the moment, there is too little space between the top two headings of the upgrade success page. We should ensure that our UI is consistent with the Pajamas UX guidelines.

![Screenshot_2020-09-11_at_12.03.52](/uploads/38c37e3eba4fe695925d474f9bfa1b67/Screenshot_2020-09-11_at_12.03.52.png)

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### User experience goal

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### What does success look like, and how can we measure that?

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### Links / references

Story points: 1.0