Add book verification for admins

Super short @houndci @georgetmayorga @cgruppioni
You should disallow `:verified` in `book_params`, like this:

```ruby
def book_params
  # what you have now, but disallowing verified
end

def admin_book_params
  book_params.merge(verified: params[:book][:verified]
end
```

If the current user is not an admin, use `book_params`; if they are, use `admin_params`.