Fixes #6678: `yii\behaviors\SluggableBehavior` will generate a new slug only when the slug attribute is empty or the source attribute is changed

Fixes #6434: Added `yii\behaviors\SluggableBehavior::immutable` to support keeping the generated slug unchanged

fix bugs of BlameableBehavior in console application. In console application, \yii\console\Application doesn't have `getUser()` method. Use BlameableBehavior in console application will cause an exception.

issue #2852

Fixes #2360: Added `AttributeBehavior` and `BlameableBehavior`, and renamed `AutoTimestamp` to `TimestampBehavior`