Turn mbstring into a required php extension

[DaSourcerer]

In Yii 1.x, there is quite some code that seeks for support from PHP's multibyte string library, silently falling back if this extension is not installed. I'd like to propose turning mbstring into a required extension, improving multibyte string handling in a save fashion and increasing code readability in the core.

As I see it, this extension isn't too exotic for mass-webhosters as a number of applications either got soft or hard dependencies on mbstring (phpMyAdmin and Drupal got soft dependencies, Horde might cease function without it and I think same goes for Joomla).

[resurtm]

+1 for hard dependency (i.e. Yii2 won't work without it). I had never seen servers without mbstring.

[qiangxue]

Agree. Let's put this as a requirement.