FEATURE: enable threading in chat DM channels (#29170)

Support threads in DMs and group chats so members can keep their conversations organized.

This change adds a new toggle switch for threads within the Chat Channel Settings screen. For new direct message channels threading is enabled by default.

We have made a decision to exclude direct message threads from the My Threads screen for now.

plugins/chat/spec/models/chat/direct_message_channel_spec.rb

@@ -52,6 +52,12 @@ RSpec.describe Chat::DirectMessageChannel do
     end
   end
 
+  describe "#threading_enabled" do
+    it "defaults to true" do
+      expect(channel.threading_enabled).to be(true)
+    end
+  end
+
   describe "#title" do
     subject(:title) { channel.title(user) }