Add contest duration & complete ranking mode implementation

config/locales/en.yml

@@ -53,6 +53,8 @@ en:
         projected_time: Projected time
         remaining_pieces: Remaining pieces (not completed puzzle)
       contest:
+        duration: Duration
+        duration_description: Format h:mm or hh:mm
         lang: Language for the public scoreboard
         name: Name
         offline_form: Enable the offline participation form
@@ -114,6 +116,8 @@ en:
               not_a_number: This is not an integer
         contest:
           attributes:
+            duration:
+              invalid: Invalid duration
             name:
               blank: The contest name cannot be empty
         contestant: