Fixes #3358: Removed automatic CSRF meta tag generation by `View`. Added `Html::csrfMetaTags()` and its call to main layout files

fixes #3245, #3244

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

This reverts commit a5846fa2, reversing
changes made to 2a6e064a.

MimeTypeController generates a map of file extensions to MIME types

It uses `mime.types` file from apache http located under
http://svn.apache.org/viewvc/httpd/httpd/trunk/docs/conf/mime.types?view=markup

This file has been placed in the public domain for unlimited
redistribution,
so we can use it and ship it with Yii.

see also yiisoft/yii#2851 and yiisoft/yii#2857

it correct work with integer keys of list

```getValue``` didn't check if the property on an object exists so wouldn't return the default value.

Fixed

No changelog line since feature was introduced after beta

Fixes #2906: Added support for using conditional comments for js and css files registered through asset bundles and Html helper