Configuration Wizard: MailChimp input error identification

[afercia]

Splitting this out from Yoast/yoast-components#843
See also https://github.com/Yoast/design/issues/323

All the inputs that require a specific format should use an input validation mechanism and provide proper feedback to users.

In the Mailchimp email field:

• the entered value is validated, however the feedback is insufficient
• when there's an error, the input field is not clearly identified
• the item in error is not styled
• even if it was styled, color alone is not sufficient: it needs an additional shape
• the error text should be positioned above the field, see Refactor schema, add Person schema wordpress-seo#12456 (comment)
• it is still possible to move to the next step even after entering a clearly invalid value

Good news:

• the visible message area is an ARIA live region so error (and success) messages are announced to assistive technologies
• there are proper suggestions for correction