Bug 28930: Keep cardnumber be editable after server side form validation fails during...

Bug 28930: Keep cardnumber be editable after server side form validation fails during self registration.

Test plan:

- Enable the system preference PatronSelfRegistration
- Make sure  PatronSelfRegistrationBorrowerUnwantedField does not
  contain cardnumber.
- Set the system preference  GDPR_Policy to "Enforce".
- In the OPAC click "Log in to your account" and "Register here".
- Fill out the form with valid data, but make sure you leave the
  checkbox under "GDPR consent" unchecked.
- Submit the form
- You will get back the form with the error message "You have not filled
  out all required fields. Please fill in all missing fields and
  resubmit."
- Without patch: cardnumber is not editable, and there is no input
  element for borrower_cardnumber (so the cardnumber will be lost when
  resubmitting the form)
- With patch: cardnumber is editable.

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Joonas Kylmälä <joonas.kylmala@iki.fi>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>