Extract ActionText::Record

Permit applications to hack in custom DB config for ActionText::RichText until AT has first-class multi-DB support:

    ActiveSupport.on_load(:action_text_record) do
      connects_to reading: :action_text_replica, writing: :action_text_primary
    end

actiontext/app/models/action_text/record.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+module ActionText
+  class Record < ActiveRecord::Base #:nodoc:
+    self.abstract_class = true
+  end
+end
+
+ActiveSupport.run_load_hooks :action_text_record, ActionText::Record

actiontext/app/models/action_text/rich_text.rb

@@ -5,7 +5,7 @@ module ActionText
   # It also holds all the references to the embedded files, which are stored using Active Storage.
   # This record is then associated with the Active Record model the application desires to have
   # rich text content using the +has_rich_text+ class method.
-  class RichText < ActiveRecord::Base
+  class RichText < Record
     self.table_name = "action_text_rich_texts"
 
     serialize :body, ActionText::Content