fixes #6691

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

Fixes #6589: updated code style guide to mention else after return, adjusted code involved in the issue

Fixes #6632: `yii\di\Container::get()` did not handle config parameter correctly when it is passed as a constructor parameter

A back quote sign in docblock missed.

introduced in https://github.com/yiisoft/yii2/commit/bf206efc66bc47b693271359e433679c1618189b

Fixes #6427: In case of invalid route web application now throws exception with "Page not found" instead of "Invalid Route"

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

Fixes #6467: `ActiveForm` will scroll to the nearest visible element when the first error input is hidden

better readability

eg. used in /var/www/app/vendor/yiisoft/yii2/base/Security.php at line 465

Signed-off-by: nabi <nabikaz@gmail.com>

Fixes #6404: advanced application template `Alert` widget was generating duplicate IDs in case of multiple flashes