feat: Allow users to mark a conversation as unread (#5924)

Allow users to mark conversations as unread.
Loom video: https://www.loom.com/share/ab70552d3c9c48b685da7dfa64be8bb3

fixes: #5552

Co-authored-by: Pranav Raj S <pranav@chatwoot.com>

app/javascript/dashboard/mixins/conversations.js

@@ -34,14 +34,6 @@ export default {
         lastNonActivityMessageFromAPI
       );
     },
-    unreadMessagesCount(m) {
-      return m.messages.filter(
-        chat =>
-          chat.created_at * 1000 > m.agent_last_seen_at * 1000 &&
-          chat.message_type === 0 &&
-          chat.private !== true
-      ).length;
-    },
     hasUserReadMessage(createdAt, contactLastSeen) {
       return !(contactLastSeen - createdAt < 0);
     },