allow extending only one class to add scopes, fixes #2146

TODO:

- [ ] adjust guide docs
- [ ] adjust README files of extensions
- [ ] finish work and fix test breaks

* 'master' of github.com:yiisoft/yii2:
  Fixes #2360: Added `AttributeBehavior` and `BlameableBehavior`, and renamed `AutoTimestamp` to `TimestampBehavior`

Fixes #2360: Added `AttributeBehavior` and `BlameableBehavior`, and renamed `AutoTimestamp` to `TimestampBehavior`

fixes #2265

Fix scrollbar in the Debug module when viewing PHP info output

Fixes #2490: `yii\db\Query::count()` and other query scalar methods now properly handle queries with GROUP BY clause

Mongodb extension refactorign

typo

test typo fix

Yii2 advanced tests fixed

Changed markdown library to cebe/markdown

- supports GFM now
- allows defining different flavors

fixes #47

Minor formatting bug in README.md

Update advanced/README.md

Changed spaces to tabs

Merge branch 'yii2_advanced_app_codeception' of github.com:Ragazzo/yii2 into Ragazzo-yii2_advanced_app_codeception

Conflicts:
	apps/advanced/README.md

fixes #2466

Update README.md

to require newly released stable version of phpdoc reflection library

Use 'Yii::$app->request->post()' instead of $_POST