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`
Suggest renaming it to `uniqueValidator` and letting it take a regular configuration (i.e. you can specify `class` and name-value pairs). If `class` is not specified, use the core unique validator.