Disallow indexing backend by default

Backend should be always totally closed for indexing by search engine robots

i18N: View path does not change if language folder exists

More docs for /yii/base/Component.php::trigger()

we should not provide one in the package but developers should commit
them when starting a new project so that all share the same versions
and only explicit composer update command will change that.
Escpecially important when deploying to production.

Fixed `Call to a member function registerAssetFiles() on a non-object` in case of wrong `sourcePath` for an asset bundle

fixes #1469

Add extensions support to yii\twig\ViewRenderer

related to #1482

fixes #1293

also filled framework CHANGELOG with relevant changes since alpha

cleanup after #1479

fixes #1480

Removed compatibility code for PHP < 5.4 in yii\web\Session