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.
