♻️(backend) refacto email invitation

Remove email invitation from Invitation model
to be able to use it in other context.
We add it in utils.py instead, and it will be called
from the viewset.
We add the document_id to link to the document from
the mail.

src/backend/core/api/serializers.py

@@ -262,7 +262,6 @@ class InvitationSerializer(serializers.ModelSerializer):
                 "Only owners of a document can invite other users as owners."
             )
 
-        user.language = request.headers.get("Content-Language", "en")
         attrs["document_id"] = document_id
         attrs["issuer"] = user
         return attrs