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

This is based on the code from Yii 1.1.14 by Ivo Kund.
update changelog for 4317
Fixes #4298: Renamed framework/messages/pt-PT to framework/messages/pt in order to allow fallbacks
avoid confusion about uploadRequired
Fixes #4297: Added check for DOM extension to requirements
Merge branch 'master' of git://github.com/yiisoft/yii2 into 4317-absolute-auth-timeout

Conflicts:
	framework/CHANGELOG.md
Fixes #4318: `yii\helpers\Html::ul()` and `ol()` will return an empty list tag if an empty item array is given
`yii\mongodb\log\MongoDbTarget` log target added
Fixes #4310: code cleanup
Fixes #4295: reworked message extraction for PO files
improved docs about goBack()

fixes #4343
Fixes #4331: `yii\helpers\Url` now uses `UrlManager` to determine base URL when generating URLs