CSS Coding standards: Use double quotes for values

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As per the ​CSS Coding standards:

Values:
Use double quotes rather than single quotes, and only when needed, such as when a font name has a space.

Specifically, for CSS generated content (especially icons) there are good reasons to always use double quotes, see [34011] / #33786. Even when the icons don't have to be swapped for RTL, using double quotes it's always a good practice.

I'd also propose to update the Coding Standards page to explicitly mention values for the content property.

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43220.2.diff​ fixes also some indentation and double spaces in the touched files.

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In 42641:

Administration: CSS coding standards: use double quotes for values that require quotes.

As per the WordPress CSS coding standards, CSS properties with values that require quotes must use double quotes.
Also fixes some indentation and spacing.

Fixes #43220.