Refactor customer registration emoji validation

We recently introduced validation against emojis on the frontend. During the implementation of , we realised it would've been less tedious work to already abstract the validation logic (gitlab-org/customers-gitlab-com!5006), in order to reduce the amount of later refactoring. We later introduced VeeValidate to do the heavy lifting for custom form validation.

This issue has been changed from abstracting the logic to refactoring the validation for the customer sign-up / registration page.

Discussion for approach for custom validation - #4644 (closed)

Proposed solution

• Apply VeeValidate similar to how we do it for customer details edit
• Delete all emoji-specific logic

Description	MR
Refactor customer registration emoji validation	tbd