Add or remove book copies to the library

[rody]

As a library admin, I want to be able to add a copy to an existing book in the library. An admin should also be able to remove a copy without affecting the Book object.

Criterias:

• A book copy must be linked to a book.
• use standard platform UI

The required fields are: -Book foreign key on Book Copy -Status (set by default as Available on creation) -Title (automatically added via Book)

Acceptance Criteria:

• [x] Admin can add a Book Copy to a Book
• [x] Admin can delete a Book Copy from a Book
• [x] Non-admin cannot add or delete a Book Copy
• [x] It should not be possible to create a Book Copy that does not have a Book: If an admin tries to make a Book Copy manually, validation cancels that action
• [x] If a Book is deleted, it's Book Copies are also removed