Refactor invoice PDF generation, download and delivery

- Remove `Que::Mailer` (#895)
- Extract controllers
- Extract translations
- Convert HAML to ERB
- Add mailer preview
- Improve UI
- Remove unused routes
- Add tests

test/system/admin_area/invoices_test.rb

@@ -1,9 +1,13 @@
 require 'test_helper'
 
 class AdminAreaInvoicesTest < ApplicationSystemTestCase
+  include ActionMailer::TestHelper
+
   setup do
     sign_in users(:admin)
     @invoice = invoices(:one)
+
+    ActionMailer::Base.deliveries.clear
   end
 
   def test_cancels_an_invoice
@@ -17,4 +21,23 @@ class AdminAreaInvoicesTest < ApplicationSystemTestCase
     assert @invoice.cancelled?
     assert_text 'Invoice has been cancelled'
   end
+
+  def test_invoice_delivery_form_is_pre_populated_with_billing_email_of_a_registrar
+    assert_equal 'billing@bestnames.test', @invoice.buyer.billing_email
+    visit new_admin_invoice_delivery_url(@invoice)
+    assert_field 'Recipient', with: 'billing@bestnames.test'
+  end
+
+  def test_delivers_an_invoice
+    visit admin_invoice_url(@invoice)
+    click_on 'Send'
+    fill_in 'Recipient', with: 'billing@registrar.test'
+    click_on 'Send'
+
+    assert_emails 1
+    email = ActionMailer::Base.deliveries.first
+    assert_equal ['billing@registrar.test'], email.to
+    assert_current_path admin_invoice_path(@invoice)
+    assert_text 'Invoice has been sent'
+  end
 end