When inviting an instructeur, the code first looked up an existing
instructeur using the normalized (downcased) email address–but then
tried to promote it using the non-normalized version, which failed.
This fixes the issue by always using the normalized email.
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.
