Fix translation keys and participant pluralization in list_with_period.html.twig

Updated French translations for "Participants" and improved pluralization handling in accompanying_period keys. Modified list_with_period.html.twig to dynamically translate "Participants" based on the count of current participations.

src/Bundle/ChillPersonBundle/translations/messages+intl-icu.fr.yaml

@@ -13,6 +13,14 @@ Requestor: >-
         neutral {Demandeur·euse}
     }
 
+accompanying_period:
+    Participants_without_count: >-
+        {count, plural,
+            =0 {Participant}
+            =1 {Participant}
+            other {Participants}
+        }
+
 person:
     from_the: depuis le
     And himself: >-