Feat: add email additional partial entry for signup box

What does this MR do and why?

This MR adds an additional partial renderer to the signup box and an additional class name for the email form section.

Background

On Jihu side, we need to verify if the user input email exists or not, just like the username validator, but for now, the section for email does not have:

1. Validation message section
2. A selector like js-validate-username for selecting the username input box.

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