Avoid reloading the zone after creating an element

Closes #377 (closed)

When creating an element without slim_output, three extra queries were made after creating an element, to retrieve the zone, the image and the image server. Those were caused by the zone_id optimization made for slim_output (!836 (merged)) and causes elements with null zones to be returned with a Postgres cluster.

Benchmarks with manage.py gunicorn and without a cluster:

Endpoint	master	This branch
CreateElement	105ms	85ms
CreateElement with slim_output	57ms	58ms