Make asking for missing pieces in forms optional

config/locales/en.yml

@@ -59,6 +59,7 @@ en:
         duration: Duration
         duration_description: Format h:mm or hh:mm
         lang: Language for the public scoreboard
+        missing_pieces: Ask for missing pieces for completed puzzles
         name: Name
         offline_form: Enable the offline participation form
         offline_form_description: Offline participants will have to fill the form by providing an image taken of the puzzle before starting solving it, and validate their finish time with an upload of an image of the completed puzzle