DEV: Start threading for chat (#20060)

Adds hidden `enable_experimental_chat_threaded_discussions`
setting which will control whether threads show in the UI,
alongside the `ChatChannel.threading_enabled` boolean column,
which does the same. The former is a global switch for this
feature, while the latter can be used to allow single channels
to show this new functionality if the site setting is true.

Neither setting impacts whether `ChatThread` records (which will
be added in a future PR) will be created, they will always be
made regardless.

plugins/chat/db/migrate/20230130053144_add_threading_enabled_to_chat_channels.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class AddThreadingEnabledToChatChannels < ActiveRecord::Migration[7.0]
+  def change
+    add_column :chat_channels, :threading_enabled, :boolean, default: false, null: false
+  end
+end