Add ability to easily mention users in chat by right clicking username

osu.Game/Localisation/ChatStrings.cs

@@ -19,6 +19,11 @@ namespace osu.Game.Localisation
         /// </summary>
         public static LocalisableString HeaderDescription => new TranslatableString(getKey(@"header_description"), @"join the real-time discussion");
 
+        /// <summary>
+        /// "Mention"
+        /// </summary>
+        public static LocalisableString MentionUser => new TranslatableString(getKey(@"mention_user"), @"Mention");
+
         private static string getKey(string key) => $"{prefix}:{key}";
     }
 }