Allow rendering template outside of the device context

[mraerino]

API call to render for any device:

curl -X 'POST' \
  'https://netbox.freifunk-duesseldorf.de/api/extras/config-templates/1/render/?format=txt' \
  -H 'Content-Type: application/json' \
  -H 'Authorization: Token <TOKEN>' \
  -d '{
  "device_id": 21
}'