Fix #4630: automatic generating of unique slug value to `yii\behaviors\Sluggable`

Fixes #4630
Added automatic generating of unique slug value to `yii\behaviors\Sluggable`
General config to create validator is:
```
[
    'class' => 'yii\validators\UniqueValidator',
    'targetAttribute' => '...',
    'filter' => [...]
    ...
]
```

'class' key is filled up automatically, `targetAttribute` setup does not needed. 
So if developer even whould like to adjust something it most probably will be `UniqueValidator::filter` value.