Move include to prepended block to fix emails

Due to some magical interaction between ActiveSupport::Concern and
ActionMailer::Base, we'll need to move them into the prepended block,
otherwise those email methods cannot be properly promoted as class
methods for Notify.

ee/app/mailers/ee/notify.rb

@@ -3,10 +3,15 @@ module EE
     extend ActiveSupport::Concern
     extend ::Gitlab::Utils::Override
 
-    include ::Emails::AdminNotification
-    include ::Emails::CsvExport
-    include ::Emails::ServiceDesk
-    include ::Emails::Epics
+    # We need to put includes in prepended block due to the magical
+    # interaction between ActiveSupport::Concern and ActionMailer::Base
+    # See https://gitlab.com/gitlab-org/gitlab-ee/issues/7846
+    prepended do
+      include ::Emails::AdminNotification
+      include ::Emails::CsvExport
+      include ::Emails::ServiceDesk
+      include ::Emails::Epics
+    end
 
     attr_reader :group
 

ee/spec/mailers/emails/admin_notification_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Emails::AdminNotification do
+  it 'adds email methods to Notify' do
+    subject.instance_methods.each do |email_method|
+      expect(Notify).to be_respond_to(email_method)
+    end
+  end
+end

ee/spec/mailers/emails/csv_export_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 require 'email_spec'
 
@@ -5,6 +7,12 @@ describe Emails::CsvExport do
   include EmailSpec::Matchers
   include_context 'gitlab email notification'
 
+  it 'adds email methods to Notify' do
+    subject.instance_methods.each do |email_method|
+      expect(Notify).to be_respond_to(email_method)
+    end
+  end
+
   describe 'csv export email' do
     let(:user) { create(:user) }
     let(:empty_project) { create(:project, path: 'myproject') }

ee/spec/mailers/emails/epics_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Emails::Epics do
+  it 'adds email methods to Notify' do
+    subject.instance_methods.each do |email_method|
+      expect(Notify).to be_respond_to(email_method)
+    end
+  end
+end

ee/spec/mailers/emails/service_desk_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Emails::ServiceDesk do
+  it 'adds email methods to Notify' do
+    subject.instance_methods.each do |email_method|
+      expect(Notify).to be_respond_to(email_method)
+    end
+  end
+end