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`
If model is updated, have `slugAttribute` filled up and none of slug source attributes are not 'dirty', there is no need to regenerate and revalidate slug.
Generating unique slug via iteration may take significant time, so such optimization is crucial.