Starting from Rails 5.1, `form_with` behavior is to generate remote
forms by default.
However with Turbolinks disabled, the form gets sent, but nothing is set
up to handle the server response (like replacing the content of the
page).
So we have two choices: either enable a global config option that makes
`form_with` generate non-remote forms, or do it explicitely on each
form. I chose the explicit way, so that developers expecting the usual
remote behavior of `form_with` are not surprised.
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.
