Forms: Fix button padding

[dhasilva]

Fixes FORMS-468

Proposed changes:

• Compute the padding values from a button to set the style variable for form button padding instead of using computed input padding values

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
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Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

• Use a theme that sets different padding values for left and right on input elements of type "text", or set a custom CSS to force this
• Add a form and publish it

Before the PR:

• The Submit button would have weird padding values

After the PR:

• The Submit button should have the same padding as other buttons, usually centered

Before	After

[github-actions]

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bin/jetpack-downloader test jetpack fix/forms-button-padding

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[github-actions]

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Code Coverage Summary

This PR did not change code coverage!

That could be good or bad, depending on the situation. Everything covered before, and still is? Great! Nothing was covered before? Not so great. 🤷

Full summary · PHP report · JS report

[simison]

Looks good overall. FYI @CGastrell this PR since he was looking at switching to core buttons.

[simison]

Oh and check that the animating cirlce still works and looks as expected when it gets injected on submit!