Fix conflict between normalize and array_attribute macros.

Fix the `normalize` and `array_attribute` macros conflicting with each
other on the WikiPage model. This meant code like
`wiki_page.other_names = "foo bar"` didn't work. Both macros defined a
`other_names=` method, but one method overrode the other.

The fix is to use anonymous modules and prepend so we can chain method
calls with super.

app/models/application_record.rb

@@ -4,6 +4,7 @@ class ApplicationRecord < ActiveRecord::Base
   include Deletable
   include Mentionable
   include Normalizable
+  include ArrayAttribute
   extend HasBitFlags
   extend Searchable
 
@@ -157,37 +158,6 @@ class ApplicationRecord < ActiveRecord::Base
     end
   end
 
-  concerning :AttributeMethods do
-    class_methods do
-      # Defines `<attribute>_string`, `<attribute>_string=`, and `<attribute>=`
-      # methods for converting an array attribute to or from a string.
-      #
-      # The `<attribute>=` setter parses strings into an array using the
-      # `parse` regex. The resulting strings can be converted to another type
-      # with the `cast` option.
-      def array_attribute(name, parse: /[^[:space:]]+/, cast: :itself)
-        define_method "#{name}_string" do
-          send(name).join(" ")
-        end
-
-        define_method "#{name}_string=" do |value|
-          raise ArgumentError, "#{name} must be a String" unless value.respond_to?(:to_str)
-          send("#{name}=", value)
-        end
-
-        define_method "#{name}=" do |value|
-          if value.respond_to?(:to_str)
-            super value.to_str.scan(parse).map(&cast)
-          elsif value.respond_to?(:to_a)
-            super value.to_a
-          else
-            raise ArgumentError, "#{name} must be a String or an Array"
-          end
-        end
-      end
-    end
-  end
-
   def warnings
     @warnings ||= ActiveModel::Errors.new(self)
   end