add configurable Navbar defined in module config

In current situation the NavBar have fixed useless links. 
Its more flexible to make it configurable and let user add it in configuration as shown below
```php
       'admin' => [
            'class' => 'mdm\admin\Module',
            'layout' => 'left-menu',
            'navbar'=>[
                ['label'=>Yii::t('app', 'Manage Forum'), 'url'=>['/forum/setting/forum']],
                ['label'=>Yii::t('app', 'Member groups'), 'url'=>['/forum/setting/group']],
            ]
        ], 


```
```php
public $navbar;
```