DEV: Move discourse-chat to the core repo. (#18776)

As part of this move, we are also renaming `discourse-chat` to `chat`.

plugins/chat/app/models/concerns/chatable.rb

@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+module Chatable
+  extend ActiveSupport::Concern
+
+  def chat_channel
+    channel_class.new(chatable: self)
+  end
+
+  def create_chat_channel!(**args)
+    channel_class.create!(args.merge(chatable: self))
+  end
+
+  private
+
+  def channel_class
+    case self
+    when Category
+      CategoryChannel
+    when DirectMessageChannel
+      DMChannel
+    else
+      raise "Unknown chatable #{self}"
+    end
+  end
+end