instructeurs: always eager load the user relationship

Now that `Instructeur.email` is merely an alias to `instructeur.user.email`,
and we changed every occurence of `instructeurs.pluck(:email)` to
`instructeurs.map(&:email)`, the new version using `map` may cause N+1 queries
if the users have not been preloaded.

It makes sense to always preload the user when fetching an Instructeur:

- Instructeur and User have a strongly coupled relationship
- It avoids N+1 queries everywhere in the app

Of course fetching an instructeur without needing its user will now do an
unecessary fetch of the associated user. But it seems better than leaving
a risk of N+1 queries in many places.

app/models/instructeur.rb

@@ -24,6 +24,8 @@ class Instructeur < ApplicationRecord
 
   has_one :user
 
+  default_scope { eager_load(:user) }
+
   def self.by_email(email)
     Instructeur.eager_load(:user).find_by(users: { email: email })
   end