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.
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.
Make it more reusable, by:
- Renaming `attachment/update` to `attachment/edit`
- Refactoring the CSS styles into their own stylesheet
- Allow to specify the 'accept' option
