fixes #4012

Added support for transaction isolation levels

add translation for intro-upgrade-from-v1 [skip ci]

[ci skip]

Added ability to `yii\widgets\Menu` to encode each item's label separately

Added ability to `yii\bootstrap\Tabs` to encode each `Tabs::items['label']` separately

fixes #3220

thanks to @Ragazzo
fixes #3799

Fix translate

Fix typo in input-validation.md guide [skip ci]

made it consistent with the GD function.

See also https://github.com/yiisoft/yii/pull/3406

Minor changes in Estonian translation

Spanish guide structure-controllers.md [skip ci]