Remove circular dependency on Redactable in migration

Gitlab::BackgroundMigration::RedactLinks was prepending
EE::Gitlab::BackgroundMigration::RedactLinks, while
EE::Gitlab::BackgroundMigration::RedactLinks was trying to include
Gitlab::BackgroundMigration::RedactLinks::Redactable. Ruby 2.5.3 failed
with an uninitialized constant
(https://gitlab.com/gitlab-org/gitlab-ee/-/jobs/118388511).

lib/gitlab/background_migration/redact_links.rb

 # frozen_string_literal: true
 # rubocop:disable Style/Documentation
 
+require_relative 'redact_links/redactable'
+
 module Gitlab
   module BackgroundMigration
     class RedactLinks
-      module Redactable
-        extend ActiveSupport::Concern
-
-        def redact_field!(field)
-          self[field].gsub!(%r{/sent_notifications/\h{32}/unsubscribe}, '/sent_notifications/REDACTED/unsubscribe')
-
-          if self.changed?
-            self.update_columns(field => self[field],
-                                "#{field}_html" => nil)
-          end
-        end
-      end
-
       class Note < ActiveRecord::Base
         include EachBatch
-        include Redactable
+        include ::Gitlab::BackgroundMigration::RedactLinks::Redactable
 
         self.table_name = 'notes'
         self.inheritance_column = :_type_disabled
@@ -27,7 +16,7 @@ module Gitlab
 
       class Issue < ActiveRecord::Base
         include EachBatch
-        include Redactable
+        include ::Gitlab::BackgroundMigration::RedactLinks::Redactable
 
         self.table_name = 'issues'
         self.inheritance_column = :_type_disabled
@@ -35,7 +24,7 @@ module Gitlab
 
       class MergeRequest < ActiveRecord::Base
         include EachBatch
-        include Redactable
+        include ::Gitlab::BackgroundMigration::RedactLinks::Redactable
 
         self.table_name = 'merge_requests'
         self.inheritance_column = :_type_disabled
@@ -43,7 +32,7 @@ module Gitlab
 
       class Snippet < ActiveRecord::Base
         include EachBatch
-        include Redactable
+        include ::Gitlab::BackgroundMigration::RedactLinks::Redactable
 
         self.table_name = 'snippets'
         self.inheritance_column = :_type_disabled

lib/gitlab/background_migration/redact_links/redactable.rb

+# frozen_string_literal: true
+# rubocop:disable Style/Documentation
+
+module Gitlab
+  module BackgroundMigration
+    class RedactLinks
+      module Redactable
+        extend ActiveSupport::Concern
+
+        def redact_field!(field)
+          self[field].gsub!(%r{/sent_notifications/\h{32}/unsubscribe}, '/sent_notifications/REDACTED/unsubscribe')
+
+          if self.changed?
+            self.update_columns(field => self[field],
+                                "#{field}_html" => nil)
+          end
+        end
+      end
+    end
+  end
+end