Tidy up high-score-boxes interactive

[JackMorganNZ]

Description

• Stop boxes from having sides touching.
• Use box images in the base-files folder, rather than make another copy. Also add new images to this folder.
• Keep 'Submit' and 'Restart' buttons consistent in their formatting.
• Store all JS variables within an object for the interactive.
• Tidy HTML and CSS files.

Related PRs: #332 #390

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