Users can accept terms during registration

What does this MR do?

This adds a checkbox to the registration form:

The checkbox is shown when terms are enforced in the admin panel of the instance. The checkbox links to /-/users/terms which is now accessible to unauthenticated users. The link opens in a new tab so the users can close it and continue their registration.

This new checkbox is required when the terms are enforced for the installation. When the user checked the checkbox during registration, they will not be required to accept the terms again later.

When the terms page /-/users/terms is viewed by an unauthenticated user, the buttons to Accept, Decline or Continue are hidden. So is the profile menu at the top:

Since the new checkbox mentions both ToS and Privacy Policy, the form in the admin area has been updated to reflect that:

While I was there I added the missing fieldset-form-class.

Why was this MR needed?

Because the current registration form was not in compliance with GDPR: https://gitlab.com/gitlab-org/gitlab-ee/issues/6262

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/6262 Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/47621

EE-port in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6060