WIP merge ActiveRelation into ActiveQuery

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
made createQuery() config param optional
merged ActiveQueryInterface and ActiveRelatioInterface
updated docs about active relation
Merge pull request #2497 from yiisoft/merge-query-and-relation2

WIP merge ActiveRelation into ActiveQuery