Refactored ActiveRecord::find() so that find(null) will also return an ActiveRecord instance instead of ActiveQuery.

so information about changed values is available in `afterSave`-event.

fixes #2892

fixed some urls in guide; moved elasticsearch png file to proper location, to be seen by apidoc generator

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 #2370

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

fixes #2281

fixes #1864

fixes #1993

issue #1313

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

this panel shows all logged elasticsearch queries and allows to execute
them to see the results.