Arrows to display the previous and next files.

2559 pjax filter

Fixing #2556: finished theming docs, emphasized on the fact that themes are non-redistributable and application-specific

replaced keyword "yii" with "yii2" in composer.json files (fixes #2557)

Add $transliteration for Vietnamese

Fixed typo in upgrade guide

Please review and merge it,
Thank advance

* origin/2559-pjax-filter:
  fix formatting

Fixes #1535: Improved `yii\web\User` to start session only when needed. Also prepared it for use without session.