Fixes #3979: Alert widget in advanced app is now able to display multiple flash messages of the same type

Fixes #3535: Syntax changes

Update a tip and an example

- add a tip that rbac.php is needed to create manually when using PhpManager.
- update that RbacController example needs to import Yii namespace.

Fixes #3956: Flash messages set via `Yii::$app->session->setFlash()` will be removed only if they are accessed

- Removed `form_begin`, `form_end`
- Added `use()` and `ViewRenderer::uses` that are importing classes and namespaces
- Added widget dynamic functions `*_begin`, `*_end`, `*_widget`, `widget_end`
- Added more tests

Translate docs(concept-components.md) to russian.

Enh: Sphinx Gii generator created

Fixes Twig lexerOptions throwing exception (#3877) 

Fixes #3939: `\yii\Inflector::slug()` improvements

- Added protected `\yii\Inflector::transliterate()` that could be replaced with custom translit implementation.
- Added proper tests for both intl-based slug and PHP fallback.
- Removed character maps for non-latin languages.
- Improved overall slug results.
- Added note about the fact that intl is required for non-latin languages to requirements checker.

Twig extensions were added twice

Update getting-started.md

Change git clone git@github.com:yiisoft/yii2-app-basic.git to 
git clone https://github.com/yiisoft/yii2-app-basic

Fixed #3934: Added attr() method instead prop()

Typo in guide

Fixed typo in validation guide

Enh: MongoDB migration support

Thai Translation