[WIP] New asset management

Fixes https://github.com/yiisoft/yii2/issues/4454

## Current status

* Basic app can work after running the following bower commands manually:

```
bower install yii2#master --save
bower install yii2-bootstrap#master --save
bower install yii2-debug#master --save
```

## Tasks to be done

* fix `yii2-jui` extension
* fix `yii2-authclient` extension
* support automatic bower package installation (via bowerphp) when using composer to install an extension
* documentation