Fixes #2491: Added support for using the same base class name of search model and data model in Gii [skip ci]

fixes #2367

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

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

- supports GFM now
- allows defining different flavors

fixes #47

Fixes #1881: Improved `yii\bootstrap\NavBar` with `containerOptions`, `innerContainerOptions` and `renderInnerContainer`

Fixes #1186: Changed `Sort` to use comma to separate multiple sort fields and use negative sign to indicate descending sort

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