Forms: Better handle custom submission message formatting

[dhasilva]

Fixes FORMS-310

Proposed changes:

• Keeps default paragraph margins for custom response messages
• Use pre-wrap white-space instead of using wpautop function, allowing vertical spacing control to work as expected, respecting multiple lines

Other information:

• Have you written new tests for your changes, if applicable?
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Does this pull request change what data or activity we track or use?

Testing instructions:

• Add a form with a custom message on submission (form block > Action after submit > "Show a custom text message")
• Make sure the custom text message has more than one paragraph by using newlines
• Publish the post and submit the form
• Check that the vertical spacing between lines is respected.

Editor:

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bin/jetpack-downloader test jetpack fix/forms-success-message-formatting

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Code Coverage Summary

Coverage changed in 1 file.

File	Coverage	Δ%	Δ Uncovered
projects/packages/forms/src/contact-form/class-contact-form.php	601/1123 (53.52%)	-0.05%	2 ❤️‍🩹

Full summary · PHP report · JS report

[ilonagl]

Nice!

[simison]

Let's also add text-wrap: pretty; which has pretty wide support and results in nice, balanced multi-line flow. It often is applied already by modern themes, but if not then ours fills the gap for the rest of the themes.

[simison]

For new CSS classes, I'm thinking we could start use jetpack_forms_ prefix instead, WDYT?