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/integration/admin_area/invoices_test.rb

@@ -6,8 +6,14 @@ class AdminAreaInvoicesIntegrationTest < ApplicationIntegrationTest
     sign_in users(:admin)
   end
 
-  def test_download_invoice_pdf
-    get admin_invoice_download_pdf_path(@invoice)
+  def test_downloads_invoice
+    assert_equal 1, @invoice.number
+
+    get download_admin_invoice_path(@invoice)
+
     assert_response :ok
+    assert_equal 'application/pdf', response.headers['Content-Type']
+    assert_equal 'attachment; filename="invoice-1.pdf"', response.headers['Content-Disposition']
+    assert_not_empty response.body
   end
 end