Artwork model ok

Modèle des oeuvres : 
- scaffold, 
- nouvelles routes : 

organization_artworks_path	 
== GET	 /organizations/:organization_id/artworks(.:format)	 artworks#index
== POST	 /organizations/:organization_id/artworks(.:format)	 artworks#create
new_organization_artwork_path	 
== GET	 /organizations/:organization_id/artworks/new(.:format)	 artworks#new
edit_organization_artist_artwork_path	 
== GET	 /organizations/:organization_id/artists/:artist_id/artworks/:id/edit(.:format)	 artworks#edit
organization_artist_artwork_path	 
== GET	 /organizations/:organization_id/artists/:artist_id/artworks/:id(.:format)	 artworks#show
== PATCH	 /organizations/:organization_id/artists/:artist_id/artworks/:id(.:format)	 artworks#update
== PUT	 /organizations/:organization_id/artists/:artist_id/artworks/:id(.:format)	 artworks#update
== DELETE	 /organizations/:organization_id/artists/:artist_id/artworks/:id(.:format)	 artworks#destroy

- url imbriquées pour l'édition et non imbriquées pour la création
- ajout de liens pour ajouter une oeuvre:
-- dans le dashboard
-- dans la show view d'un artiste

- ajout d'une relation artwork / organization via la table Artists (pour pouvoir écrire organization.artworks)