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/app/models/chat_channel.rb

@@ -164,6 +164,7 @@ end
 #  user_count_stale             :boolean          default(FALSE), not null
 #  slug                         :string
 #  type                         :string
+#  threading_enabled            :boolean          default(FALSE), not null
 #
 # Indexes
 #