see https://github.com/yiisoft/yii2/commit/0cfd41a21a8d89cc577590a253893542e60d43ce#commitcomment-5238567
issue #2233

Fixes #1412: `FileValidator` still triggers `uploadRequired` error in some case when `skipOnEmpty` is true and no upload is provided

fixes #2233

Improved `QueryBuilder::buildLimit()` to support big numbers

Revert "Fixes #1856: The default CSS class for hint generated by `ActiveField` is changed to `help-block`"

This reverts commit 7ae4695d.

Fixes #2212: `yii\gridview\DataColumn` generates incorrect labels when used with nosql DB and there is no data

Fixes #706: Added `yii\widgets\Pjax` and enhanced `GridView` to work with `Pjax` to support AJAX-update