Docs about mail debugging added
fixed breaking Gii preview when file is unchanged

fixes #3693
fixed test break
added CHANGELOG for #3693
added explicit comment about file transport to mail config

fixes #3694
ES: fixed issue with storing empty records

fixes #3587
fixed test breaks
finished tutorial on models [skip ci]
Merge branch 'master' of git://github.com/yiisoft/yii2
guide WIP [skip ci]
Fix label() method and make more templates configurable