Contact Form block: add a prefix to a generic CSS selector

Here's a little too generic CSS selector:

https://github.com/Automattic/wp-calypso/blob/35714673216d4622b73775176b828bec6c66b48e/client/gutenberg/extensions/presets/jetpack/editor-shared/help-message.scss#L2

Imported in forms block via:

https://github.com/Automattic/wp-calypso/blob/35714673216d4622b73775176b828bec6c66b48e/client/gutenberg/extensions/contact-form/components/jetpack-contact-form.jsx#L18

We should protect this CSS leaking into other Gutenberg `help-message`s.

Simple payments customizes error message styling so we should confirm that changes to this won't change behaviour there:

https://github.com/Automattic/wp-calypso/blob/35714673216d4622b73775176b828bec6c66b48e/client/gutenberg/extensions/simple-payments/help-message.scss#L3

Start with client/gutenberg/extensions/presets/jetpack/editor-shared/help-message.scss and inspect its import in client/gutenberg/extensions/contact-form/components/jetpack-contact-form.jsx. Compare the related selector in client/gutenberg/extensions/simple-payments/help-message.scss, then confirm the prefix prevents styles from leaking to other Gutenberg help messages without changing Simple Payments behavior.

Written by the indexing model from the issue text.