DB: Started update of entity loading to avoid global selects

Removes page/chpater addSelect global query, to load book slug, and
instead extracts base queries to be managed in new static class, while
updating specific entitiy relation loading to use our more efficient
MixedEntityListLoader where appropriate.

Related to #4823

app/Entities/Models/BookChild.php

@@ -18,20 +18,6 @@ use Illuminate\Database\Eloquent\Relations\BelongsTo;
  */
 abstract class BookChild extends Entity
 {
-    protected static function boot()
-    {
-        parent::boot();
-
-        // Load book slugs onto these models by default during query-time
-        static::addGlobalScope('book_slug', function (Builder $builder) {
-            $builder->addSelect(['book_slug' => function ($builder) {
-                $builder->select('slug')
-                    ->from('books')
-                    ->whereColumn('books.id', '=', 'book_id');
-            }]);
-        });
-    }
-
     /**
      * Scope a query to find items where the child has the given childSlug
      * where its parent has the bookSlug.