style(champ): add default placeholders for text & textarea champs

cc #7429

app/views/shared/dossiers/editable_champs/_text.html.haml

@@ -1,5 +1,5 @@
 = form.text_field :value,
   id: champ.input_id,
-  placeholder: champ.libelle,
+  placeholder: t(".placeholder"),
   required: champ.mandatory?,
   aria: { describedby: champ.describedby_id }

app/views/shared/dossiers/editable_champs/_textarea.html.haml

@@ -3,4 +3,5 @@
   aria: { describedby: champ.describedby_id },
   rows: 6,
   required: champ.mandatory?,
-  value: html_to_string(champ.value)
+  value: html_to_string(champ.value),
+  placeholder: t(".placeholder")

config/locales/shared.en.yml

@@ -27,6 +27,10 @@ en:
         siret:
           placeholder: "50000123456789"
           title: "The SIRET number must have exactly 14 digits"
+        text:
+          placeholder: "Your answer"
+        textarea:
+          placeholder: "Write your answer here"
       header:
         expires_at:
           brouillon: "Expires on %{date} (%{duree_conservation_totale} months after this file was created)"

config/locales/shared.fr.yml

@@ -27,6 +27,10 @@ fr:
         siret:
           placeholder: "50000123456789"
           title: "Le numéro de SIRET doit comporter exactement 14 chiffres"
+        text:
+          placeholder: "Votre réponse"
+        textarea:
+          placeholder: "Rédigez ici votre réponse"
       header:
         expires_at:
           brouillon: "Expirera le %{date} (%{duree_conservation_totale} mois après la création du dossier)"