instructeurs: fix expert creation not to use Instructeur.email

It kind of worked until now, because the email field is disabled, and
thus never accessed.

But better make it clean, by accessing an object (User) where the email
field actually exists.

app/views/instructeurs/avis/sign_up.html.haml

@@ -4,7 +4,7 @@
       %p.description= @dossier.procedure.libelle
       %p.dossier Dossier nº #{@dossier.id}
     .column
-      = form_for(Instructeur.new, url: { controller: "instructeurs/avis", action: :create_instructeur }, method: :post, html: { class: "form" }) do |f|
+      = form_for(User.new, url: { controller: "instructeurs/avis", action: :create_instructeur }, method: :post, html: { class: "form" }) do |f|
         %h1 Créez-vous un compte
 
         = f.label :email, "Email"