`aliases.php` renamed to `bootstrap.php` (+ specific for apps)

Why its needed? For example you want to use DI feature for components. Its meaningless introduce new file and include it inside entry script. One `bootstrap.php` file for everything (setting up aliases, preconfigure components, etc). Examples:

**common\bootstrap.php**:

```php
Yii::setAlias('backend', dirname(dirname(__DIR__)) . '/backend');
Yii::setAlias('booking', dirname(dirname(__DIR__)) . '/booking');
Yii::setAlias('common', dirname(__DIR__));
Yii::setAlias('console', dirname(dirname(__DIR__)) . '/console');
Yii::setAlias('representative', dirname(dirname(__DIR__)) . '/representative');
Yii::setAlias('shop', dirname(dirname(__DIR__)) . '/shop');
Yii::setAlias('uploads', dirname(dirname(__DIR__)) . '/uploads');

Yii::$container->set('yii\grid\ActionColumn', [
    'buttons' => [
        'view' => function ($url, $model) {
            return Html::a('<i class="fa fa-eye"></i>', $url, [
                'title' => Yii::t('app', 'View'),
                'class' => 'btn btn-xs blue',
                'data-pjax' => '0',
            ]);
        },
        'update' => function ($url, $model) {
            return Html::a('<i class="fa fa-edit"></i>', $url, [
                'title' => Yii::t('app', 'Update'),
                'class' => 'btn btn-xs green',
                'data-pjax' => '0',
            ]);
        },
        'delete' => function ($url, $model) {
            return Html::a('<i class="fa fa-trash-o"></i>', $url, [
                'title' => Yii::t('app', 'Delete'),
                'class' => 'btn btn-xs red',
                'data-confirm' => Yii::t('yii', 'Are you sure you want to delete this item?'),
                'data-method' => 'post',
                'data-pjax' => '0',
            ]);
        },
    ],
]);
```

**backend\bootstrap.php** (specific application bootstrap):

```php
Yii::$container->set('yii\grid\GridView', [
    'pager' => false,
]);
```

So generally this idea is evolution of `aliases.php` file ideas.