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
added missing use statement
merged ActiveQueryInterface and ActiveRelatioInterface
updated docs about active relation
fixed absolute class links in guide
Merge pull request #2497 from yiisoft/merge-query-and-relation2

WIP merge ActiveRelation into ActiveQuery