Better fix for #3443 that does not break BC and allows widget to be extensible.

`yii\web\Request::cookieValidationKey` is now automatically generated by the installation script for the basic and advanced application templates

Fixes #3443: Fixed `yii\bootstrap\Nav` and `yii\bootstrap\Dropdown` were generating wrong ids for submenus

reverts 45e4973a

fixed outdated example.

Fixes #4051: Renamed `yii\caching\GroupDependency` to `TagDependency` and added support for associating multiple tags to a single cached data item

all fine now

closes #2427

`yii\mongodb\Collection::buildRegexCondition()` added.

do not use scripting as it is disabled by default and will cause edge
case queries to fail

fixes #4187