Added automatic generating of unique slug value to `yii\behaviors\Sluggable`
Default event for `yii\behaviors\Sluggable` with `unique` enabled changed to 'insert'.
`yii\behaviors\Sluggable` optimized
`yii\behaviors\Sluggable` simplified
`yii\behaviors\Sluggable::validateSlug()` fixed to respect updating record
merge with upstream
CHANGELOG updated
`yii\behaviors\Sluggable` adjusted
Merge pull request #4675 from klimov-paul/4630-sluggable

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