enchansment of db message source

add findAndModify() method to the Collection class

Add an option to ignore pjax request on some links.

Code style fix

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

fix code style

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