Draft: Attempt to use prefetch_related

Linked to ptf-site-traduction!10 (closed) and ptf-site-ahl!4 (closed)

Things to note :

• Avoid using self.object_set.filter as it creates another query. We should prefer using the a python-based filter on a prefetched self.object_set
• Use prefetch_related when possible, as subsequent accesses create another SQL query otherwise
• Avoid using ptf.Resource.cast(). This will cleanup the ORM cache for the object.