Moved most of the user-related logic into form models
Extracted password-related code into User model methods
Allow multilevel dropdown.
Added typecast database types into php types
Fixed constructor overriding to allow array configuration
refactored file validator.
fix kazakh translate
Fixes #2260.
Merge pull request #2259 from Asetss/master

fix kazakh translation
doc improvement.
doc fix.
Used setter as field
Added ! usage for default scenario example to model guide
Typo fix
Moved forms used in frontend only to frontend
override create
skip exiting in test env.

see https://github.com/yiisoft/yii2/commit/0cfd41a21a8d89cc577590a253893542e60d43ce#commitcomment-5238567
issue #2233
Optimized forms code a bit
Fixed link in i18n docs
docs improved