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
       prepend EE::Gitlab::BackgroundMigration::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
@@ -29,7 +18,7 @@ module Gitlab
 
       class Issue < ActiveRecord::Base
         include EachBatch
-        include Redactable
+        include ::Gitlab::BackgroundMigration::RedactLinks::Redactable
 
         self.table_name = 'issues'
         self.inheritance_column = :_type_disabled
@@ -37,7 +26,7 @@ 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
@@ -45,7 +34,7 @@ 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

spec/lib/gitlab/background_migration/redact_links_spec.rb

@@ -4,6 +4,7 @@ describe Gitlab::BackgroundMigration::RedactLinks, :migration, schema: 201810141
   let(:namespaces) { table(:namespaces) }
   let(:projects) { table(:projects) }
   let(:issues) { table(:issues) }
+  let(:epics) { table(:epics) }
   let(:notes) { table(:notes) }
   let(:snippets) { table(:snippets) }
   let(:users) { table(:users) }
@@ -41,6 +42,12 @@ describe Gitlab::BackgroundMigration::RedactLinks, :migration, schema: 201810141
     snippets.create(params)
   end
 
+  def create_epic(id, params)
+    params.merge!(id: id, iid: id, title: "epic#{id}", title_html: "epic#{id}", author_id: user.id, group_id: namespace.id)
+
+    epics.create(params)
+  end
+
   def create_resource(model, id, params)
     send("create_#{model.name.underscore}", id, params)
   end
@@ -93,4 +100,11 @@ describe Gitlab::BackgroundMigration::RedactLinks, :migration, schema: 201810141
       let(:field) { :description }
     end
   end
+
+  context 'resource is Epic' do
+    it_behaves_like 'redactable resource' do
+      let(:model) { Epic }
+      let(:field) { :description }
+    end
+  end
 end