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/registrar/invoices_controller.rb

@@ -2,8 +2,6 @@ class Registrar
   class InvoicesController < BaseController
     load_and_authorize_resource
 
-    before_action :set_invoice, only: [:show, :forward, :download_pdf]
-
     def index
       params[:q] ||= {}
       invoices = current_registrar_user.registrar.invoices.includes(:items, :account_activity)
@@ -15,40 +13,20 @@ class Registrar
       end
     end
 
-    def show;
-    end
-
-    def forward
-      @invoice.billing_email = @invoice.buyer.billing_email
-
-      return unless request.post?
-
-      @invoice.billing_email = params[:invoice][:billing_email]
-
-      if @invoice.forward(render_to_string('pdf', layout: false))
-        flash[:notice] = t(:invoice_forwared)
-        redirect_to([:registrar, @invoice])
-      else
-        flash.now[:alert] = t(:failed_to_forward_invoice)
-      end
-    end
+    def show; end
 
     def cancel
       @invoice.cancel
       redirect_to [:registrar, @invoice], notice: t('.cancelled')
     end
 
-    def download_pdf
-      pdf = @invoice.pdf(render_to_string('pdf', layout: false))
-      send_data pdf, filename: @invoice.pdf_name
+    def download
+      filename = "invoice-#{@invoice.number}.pdf"
+      send_data @invoice.as_pdf, filename: filename
     end
 
     private
 
-    def set_invoice
-      @invoice = Invoice.find(params[:id])
-    end
-
     def normalize_search_parameters
       params[:q][:total_gteq].gsub!(',', '.') if params[:q][:total_gteq]
       params[:q][:total_lteq].gsub!(',', '.') if params[:q][:total_lteq]