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

app/models/invoice/pdf_generator.rb

@@ -0,0 +1,22 @@
+class Invoice
+  class PdfGenerator
+    attr_reader :invoice
+
+    def initialize(invoice)
+      @invoice = invoice
+    end
+
+    def as_pdf
+      generator = PDFKit.new(invoice_html)
+      generator.to_pdf
+    end
+
+    private
+
+    def invoice_html
+      view = ActionView::Base.new(ActionController::Base.view_paths, invoice: invoice)
+      view.class_eval { include ApplicationHelper }
+      view.render(file: 'invoice/pdf', layout: false)
+    end
+  end
+end