Fixes #1897

- Gii diff markup is now copy paste friendly
- Removed StringHelper::diff()
- Moved phpspec/php-diff dependency from yiisoft/yii2 to yiisoft/yii2-gii
changelog
Merge pull request #2173 from yiisoft/gii-diff-do-not-copy-line-numbers

Fixes #1897
Fixes #2156: `yii migrate` now automatically creates `migrations` directory if it does not exist