Update back end routes guide

[fritzmg]

The back end routes guide is a bit outdated, as it still uses Sensio\Bundle\FrameworkExtraBundle\Configuration\Route and Sensio\Bundle\FrameworkExtraBundle\Configuration\Template. Both of which are not recommended anymore.

Todo:

• Use Symfony\Component\Routing\Annotation\Route.
• Inject Twig service and parse template directly.

[fritzmg]

See #225