Updated activities table format

Renamed some columns to be more generic and applicable.
Removed now redundant book_id column.
Allowed nullable entity morph columns for non-entity activity.

Ran tests and made required changes.

app/Http/Controllers/BookSortController.php

@@ -15,10 +15,6 @@ class BookSortController extends Controller
 
     protected $bookRepo;
 
-    /**
-     * BookSortController constructor.
-     * @param $bookRepo
-     */
     public function __construct(BookRepo $bookRepo)
     {
         $this->bookRepo = $bookRepo;