API is not finished.

issue #5082

Fixes #5316: Added `startsWith()` and `endsWith()` to `yii\helpers\StringHelper`. Methods are binary-safe, multibyte-safe and optionally case-insensitive

Fixes #5058: `yii\helpers\Html::activeCheckbox()` and `activeRadio()` will generate labels by default using the corresponding attribute labels

adding and reworking the conversion of:
- ICU to PHP
- PHP to ICU
- ICU to JUI
- PHP to JUI

`$config` can now be a closure that will have the `$configInstance` passed as the first param.

updated docs.

close #4656

Fixes #4122: `Html::error()` and `Html::errorSummary()` are now accepting `encode` option. If set to false it prevents encoding of error messages

fix typo Markdown->GithubMarkdown in $flavors['gfm-comment']

Improved checks where substr_compare is used, replaced more substr + comapre cases with substr_compare

fixes #4105, close #4376

Fixes #4318: `yii\helpers\Html::ul()` and `ol()` will return an empty list tag if an empty item array is given