Fixes #2392: Added `addCssStyle()`, `removeCssStyle()`, `cssStyleFromArray()` and `cssStyleToArray()` to `Html`

fixes #2519: MessageSource removed translation messages when event handler
was bound to `missingTranslation`-event

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