When you reply to a chat message, we [always create a thread][1]. But when the channel we're in doesn't have threading enabled, the reply is _technically_ not a thread.
This changes the `in_thread?` method to check for both the presence of a `thread_id` and to ensure that the channel has `threading_enabled`.
Internal ref - t/128103/3
[1]: e6e3eaf472/plugins/chat/app/services/chat/create_message.rb (L110-L115)
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