Fix button width in repo form

[colebemis]

Update the submit button in the repo form to maintain its width when transitioning to a loading spinner.

• Add cx utility import to handle classnames.
• Wrap the button text in a span element and add the invisible class when isLoading is true.
• Add a relative class to the button element.
• Wrap the loading spinner in a span element and add the absolute, inset-0, and grid place-items-center classes to center the spinner within the button.

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