New admin function: De-activate/re-active books, authors or categories

[DavidAJohn]

At the moment, although admins can edit books and authors (and add new ones), there should also be a way to hide specific books, categories or authors from the search pages.

Of course, it should also be possible to subsequently re-activate them as well. for a category or specific book this should be reasonably simple, but de-activating an author would also involve de-activating their books as well (and vice versa).