Coding standards fixes.

nulls in batchInsert. Patch available.

Add getIsPjax method to Request, and add X-PJAX-URL header when redirect from pjax request

Fixes #2392: Added `addCssStyle()`, `removeCssStyle()`, `cssStyleFromArray()` and `cssStyleToArray()` to `Html`

Allow add a condition telling the table to avoid ambiguity error.

fixes #2519: MessageSource removed translation messages when event handler
was bound to `missingTranslation`-event

Fixes #2491: Added support for using the same base class name of search model and data model in Gii [skip ci]

fixes #2514

Fixes #2426: Changed URL creation method signatures to be consistent

WIP merge ActiveRelation into ActiveQuery