Dropdown: fix z-index for overlayed card

[RasmusKjeldgaard]

Which issue does this PR close?

This PR closes #3725

What is the new behavior?

Card is displayed on top of other content when dropdown is opened.

Does this PR introduce a breaking change?

• Yes
• No

Are there any additional context?

Checklist:

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Reminders

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When the pull request has been approved it will be merged to develop by Team Kirby.

[jakobe]

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