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

Spanish guide Query Builder [skip ci]

/docs/guide-ja/input-validations.md added [ci skip]

docs/guide/input-validation.md - typo [ci skip]

[skip ci] Update start-looking-ahead.md

[Uk] Guide corrections [skip ci]

Adding the tag titled "yii2" on Stackoverflow to Community list.

fix  mistake small  [skip ci]

Fixes #6514: advanced application SignupForm::signup() should not return User model if saving failed

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

Removed User.role from advanced application template because it was causing confusion that user could be assinged a single RBAC role and that role field is somehow tied to RBAC at all.

better readability

require PHP mcrypt extension

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

AssetBundle can not be imported because the class already exists in this namespace.

fixes #6475

fixed hhvm build.