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/controllers/concerns/deliverable.rb

@@ -0,0 +1,26 @@
+module Deliverable
+  extend ActiveSupport::Concern
+
+  included do
+    before_action :find_invoice
+  end
+
+  def new
+    authorize! :manage, @invoice
+    @recipient = @invoice.buyer.billing_email
+  end
+
+  def create
+    authorize! :manage, @invoice
+
+    InvoiceMailer.invoice_email(invoice: @invoice, recipient: params[:recipient]).deliver_now
+
+    redirect_to redirect_url, notice: t('.delivered')
+  end
+
+  private
+
+  def find_invoice
+    @invoice = Invoice.find(params[:invoice_id])
+  end
+end