Fixes #253: Fixed missing initial class `collapsed` of an `a` element in heading in `yii\bootstrap\Collapse` widget

* Use phpunit from vendor
* Ship our own phpunit as well
* Adapt test to yii 2.0.16
* Remove hhvm, add php 7.3

guide-ja revised [ci skip]

detailed docs about assets setup added

`yii\bootstrap\Html::error()` now automatically set 'help-block help-block-error' CSS class for generated tag

Methods `radioList()` and `checkboxList()` overridden at `yii\bootstrap\Html` applying Bootstrap style

Fixed `yii\bootstrap\ToggleButtonGroup` toggles dropdown for both buttons in case `split` is enabled

Fixes #219, Fixes #171: Add ability to use custom HTML in navbar-header, Add ability to use a brandImage with the navbar

Fixes #184: `yii\bootstrap\ButtonDropdown` widget did not use the correct JS plugin, was `button`, changed to `dropdown`