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

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

#2436 override the function `getAttributeLabel` in `BaseActiveRecord` to receive label of the attribute, which looks like `relatedMode.attribute`, from the related model

#2436 override the function `getAttributeLabel` in `BaseActiveRecord` to receive label of the attribute, which looks like `relatedMode.attribute`, from the related model

fixes #2370

- refactored elasticsearch AR to make type and index available in
  instantiate(), issue #1313

fixes #2281

Improved `QueryBuilder::buildLimit()` to support big numbers