Data Protection: Update and Delete

[chadell]

New features:

• CustomValidators were added for all the models defined under configuration as protected_models. This avoids direct updated of object attributes that are owned by a design, if not coming from the design updated/revert process itself
• Pre_delete signal (customized with a Middleware that injects the user session) to avoid deleting objects that are owned (full_control) by a design or referenced.
• Add a superuser bypass mechanism to overpass these data protection mechanisms
• Add a new tab for all the protected models to list which design owns each attribute or the object itself