instructeurs: make the create form not using email directly

Before the form attempted to read an email value from the Instructeur
model, and failed (because the empty Instructeur had no user yet).

We could let `Instructeur#email` return `nil` if there is no User –
but as a created Instructeur is always supposed to have a User, this
seems like a nice safeguard to keep.

So instead this commit rewrites the create form, which now doesn’t
depend on an Instructeur model. Seems easy enough for now.

spec/features/instructeurs/instructeur_creation_spec.rb

@@ -12,7 +12,7 @@ feature 'As an instructeur', js: true do
     fill_in :instructeur_email, with: instructeur_email
 
     perform_enqueued_jobs do
-      click_button 'Valider'
+      click_button 'Ajouter'
     end
   end