unread_ops: Add unread class to messages instead of a full rerender.

When user marks messages as unread, we used to rerender them but
that was not required since we only need to add `unread` class
to their `message_row` to achieve the same effect.

web/src/message_list_view.js

@@ -1587,4 +1587,13 @@ export class MessageListView {
         }
         $row.removeClass("unread");
     }
+
+    show_messages_as_unread(message_ids) {
+        const $table = rows.get_table(this.table_name);
+        const $rows_to_show_as_unread = $table.find(".message_row").filter((index, $row) => {
+            const message_id = Number.parseFloat($row.getAttribute("zid"));
+            return message_ids.includes(message_id);
+        });
+        $rows_to_show_as_unread.addClass("unread");
+    }
 }