- removed unformat methods, they do not belong here
- removed db format, which is also not purpose of this class
- refactored the whole set of methods to be simpler and better
  maintainable

More unit tests needed.

#2359 one test failed in Travis because standard medium date format is different in Travis then on my locale PC.

https://github.com/UncleCoder/yii2/commit/e43c503c2d8408f46fb4ff09330db4e5a9f1cf01

issue #3817

Improved `yii\helpers\Inflector::slug` to support more cases for Russian, Hebrew and special characters

issue #3520

verify issue #3473

This reverts commit 70b57766.