Feat(Cloud Activation Form): add validation

What does this MR do and why?

This MR adds client-side validation to the Cloud Activation Form, so users get faster feedback when trying to submit an activation code.

Screenshots or screen recordings

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Example below:

1. Checkout this branch and apply this patch: test-form-validation.patch
2. Visit the admin subscription page http://127.0.0.1:3000/admin/subscription
3. Click the activate button.

Note: accepted codes are alphanumeric of length 24 characters

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #323071 and customers-gitlab-com#3448 (closed)