Fix Customizer accordion button CSS

[sabernhardt]

• Adds box-sizing: border-box to the buttons so they do not cause a horizontal scroll (copied from patch.diff).
• Increases specificity to remove padding from headings in the Menus and Widgets panels.
• Makes the chevron icon clickable.
• Adds padding to the buttons to avoid overlapping the chevron (30 pixels, to match what the child panel headings had).
• Adds a semicolon in common.css.

Trac 62313

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[github-actions]

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Props sabernhardt.

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[github-actions]

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[apermo]

LGTM

[desrosj]

Thanks @sabernhardt. I've done some testing on this and the changes are looking good to me.

[desrosj]

Merged in https://core.trac.wordpress.org/changeset/59409.