AO3-5596 Make boxes on All Fandoms page clear properly

[slavalamp]

Pull Request Checklist

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• Have you read the contributing guidelines?
• Have you added tests for any changed functionality?
• Have you added the Jira issue number
  as the first thing in your pull request title (e.g. AO3-1234 Fix thing)
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Issue

https://otwarchive.atlassian.net/browse/AO3-5596

Purpose

Makes fandom boxes in All Fandoms have no big gaps when they're displayed in two columns and have unequal height

Testing Instructions

1. Open the All Fandoms page
2. The browser window width needs to be such that the fandoms are displayed in two columns and a box on the left needs to be taller than the box to its right, you might need to resize the window to achieve that. (For reference, for me this happens with all window widths between 880-1572px right now, but it might vary based on fandom sizes/fonts/etc)
3. There should not be a big empty space below the left box

References

n/a

Credit

slavalamp (they/them)

[Bilka2]

Thank you!