build the registration from using the userschema provided by backend

[erral]

• I signed and returned the Plone Contributor Agreement, and received and accepted an invitation to join a team in the Plone GitHub organization.
• I verified there aren't other open pull requests for the same change.
• I followed the guidelines in Contributing to Volto.
• I succesfully ran code linting checks on my changes locally.
• I succesfully ran unit tests on my changes locally.
• I succesfully ran acceptance tests on my changes locally.
• If needed, I added new tests for my changes.
• If needed, I added documentation for my changes, either in the Storybook or narrative documentation.
• I included a change log entry in my commits.

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Closes #6434

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[stevepiercy]

Minor grammar fixes.

This PR still requires technical review.

[stevepiercy]

One more instance, and I think it's GTG.

[sneridagh]

LGTM. I am willing to make it enter in 18, but I need someone to test it and give feedback.
Some Cypress tests would be great too.
Any takers?

/cc @plone/volto-team

[sneridagh]

On the other hand, we can make it for 18.1 since it's a feature.

[erral]

LGTM. I am willing to make it enter in 18, but I need someone to test it and give feedback. Some Cypress tests would be great too. Any takers?

/cc @plone/volto-team

We can check this indeed.

[erral]

I have added a cypress test that shows that the fields in the /@userschema request are rendered in the form.

I have been checking also the implementation of the registration, and I see that we are using 2 settings userEmailAsLogin and showSelfRegistration which should be coming from backend (there are indeed 2 settings in backend for them).

But using them would mean to change the REST API to expose those settings (right now the /@registry endpoint is reserved for Managers), and then change the implementation not only of the registration form but also of other links and usages of those 2 settings.

Which means it is a larger project.