Administration: Update site icon and media control button styling

[Infinite-Null]

This PR updates the appearance of the Site Icon and media upload buttons to match their functionality. The previous styling made these elements look like drop areas, confusing when users attempted to drag and drop files onto them, as they only function as clickable buttons.

Trac ticket: #47579

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[github-actions]

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Props ankitkumarshah, joedolson.

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[joedolson]

These changes look good. Have you checked whether the button-add-site-icon class is still in use? If that was the only use of that class, then we should be able to remove the related CSS.

((We can't remove button-add-media, as that's widely used by plugins and themes.)

I'd also like to see more of a compromise design: use the upload button styles, but keep the full width layout and padding.

[Infinite-Null]

Hi @joedolson, Thank you for the feedback. I have made the necessary changes and updated the PR please review it at your convenience.

Screenshot:

[joedolson]

Some additional changes that I think would help make things more consistent:

1. When an image is selected, e.g. for the background image, you end up with a small 'Remove' button and a large 'Change Image' button. I think it would be good if those were the same size, either both large or both small. I suggest small; I think that the Change Image button is different from the UPload Image button.

2. The Add Header image interface still uses the dashed outline design, but with a separate upload button. I think it would be good to make those all the same: eliminate the dashed placeholder space and use a large upload button.

These changes are otherwise good!

[joedolson]

Requested changes are noted in a comment; they're less changes to the existing code and more additions that would be helpful.