Add System emails menu & list to admin panel

We need a section to list all System Emails to view the templates and
preview an example of what would be sent.

config/routes/admin.rb

@@ -165,6 +165,11 @@ namespace :admin do
     end
   end
 
+  resources :system_emails, only: [:index] do
+    get :view
+    get :preview
+  end
+
   resources :emails_download, only: :index do
     get :generate_csv, on: :collection
   end