Merge pull request #1 from yiisoft/master

switching base
Merge pull request #2 from yiisoft/master

switching base
Merge pull request #3 from yiisoft/master

Merge pull request #3 from yiisoft/master
Исправление смысловых, орфографических и грамматических ошибок/опечаток в start-workflow.md
Update start-workflow.md
Change image links start-workflow.md
changed symbols of text from ` to ' for uzbek letters.
uzbek_translation
Update start-workflow.md

Should not be changed.
updated documentation and tests

fixes #5331
Fixes #5252: Null values are not properly handled by `RangeValidator`
Merge branch 'master' of git://github.com/yiisoft/yii2
fix missing code for infinite dropdown
Fixes #5323: Nested dropdown does not work for `yii\bootstrap\DropDown`
add BootstrapPlugin::register() to Dropdown widget
Fixes #5336: `yii\bootstrap\DropDown` should register bootstrap plugin asset
Fixes #5337
reverted color change of de87f43d707a53cd181f76e0b1160237056e277f
fixed links in application guide

fixes #5322
fixed typo

fixes #5339