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

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

fixes #2281

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

fixes #1993

Moved file and path related methods from StringHelper to FileHelper, renamed StringHelper byte methods not to be misused as string methods

- allows to have dynamic definition of attributes depended on the
  instance
- there was no real need for it to be static. Places that used it static
  have been refactored.

fixes #1318

Model::attributes() has been made static

"yii\sphinx\ActiveQuery" updated to throw exception on conflict between "asArray" and "snippetByModel" options.