Add missing french translation. (#1906)

* Add missing french translation.

Causing this error:
[React Intl] Missing message: "navigation_bar.mutes" for locale: "fr",

* Update fr.jsx

app/assets/javascripts/components/locales/fr.jsx

@@ -75,6 +75,7 @@ const fr = {
   "navigation_bar.favourites": "Favoris",
   "navigation_bar.info": "Plus d'informations",
   "navigation_bar.logout": "Déconnexion",
+  "navigation_bar.mutes": "Utilisateurs muets",
   "navigation_bar.follow_requests": "Demandes de suivi",
   "reply_indicator.cancel": "Annuler",
   "search.placeholder": "Rechercher",