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

[READY] \yii\behaviors\SluggableBehavior

Translate docs (guide/concept-properties.md) to russian.

Move the white space outside of the link