Added support for relation by array attributes

fixes #1249
fixed issue with indexBy and array valued relations
minor refactoring of FileValidator.
Fixes #3887
Fixes #4342
Merge branch 'master' of git://github.com/yiisoft/yii2 into 4342_mssql_driver_updates
Fixes #4370: fileinfo extension is required for FileValidator.
add support for array valued attr relations to unlink and link
Update CHANGELOG.md
Merge branch 'master' into array-attribute-relations

* master:
  Update output-data-widgets.md
  Fixes #4370: fileinfo extension is required for FileValidator.
  Fixes #4342
  Fixes #3887
  minor refactoring of FileValidator.
  Doc translate for rest-resources
  Better fix for Dropdown container options
  #4388: CORS filter fixes:
  Fixed test break.
  #4310: Removed `$data` from signature of `yii\rbac\ManagerInterface`
  Update .travis.yml
  Update ActiveRecordInterface.php
  Fixes #4384.
  Sphinx distributed indexes support provided
  typo fix [skip ci]
  Fixes #3611: Refactored query caching.
fixed test break

how did that manage to pass before...?