deferred validation

Added support of deferred to ```ActiveForm``` validation.
deferred validation

Added support of deferred validation.
Update yii.activeForm.js

remove empty message arrays
Update yii.activeForm.js
Update CHANGELOG.md
Update CHANGELOG.md
Update yii.activeForm.js

Refactored
Merge branch 'deferred-validation' of github.com:Alex-Code/yii2 into Alex-Code-deferred-validation

Conflicts:
	framework/CHANGELOG.md
Fixes #2315: Any operator now could be used with `yii\db\Query::->where()` operand format
Update MessageController.php

Fixed order of checking config `format` property
test and docs to be clear about #4558
Added support for arbitrary operators to Query::filterWhere, added support for arbitrary operators to Sphinx extension
Removed unnecessary code
Simplified parameter check for buildSimpleCondition()
Fixes #4571: ignore non-string attribute names for AttributeBehavior.
Added client-side image validation
Fix lines
Abort further validation if there are no files to validate
Fixes #4566: Added client validation support for image validator
renamed function name [skip ci]