Fixes #3934: Added attr() method instead prop() to get valid action url.
Sphinx Gii generator created
Docs about Sphinx Gii generator added
Update start-databases.md
Merge remote-tracking branch 'chinesization/master'
Update start-forms.md
Merge pull request #3963 from joni-jones/master

Fixed #3934: Added attr() method instead prop()
Fixed CHANGELOG [skip ci]
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
Merge pull request #3967 from SanderLi/patch-1

Update getting-started.md
Merge pull request #3767 from grachov/patch-1

Twig extensions were added twice
Updated twig extension changelog
Fixes #3968: Messages logged in shutdown functions are not handled
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.
Merge pull request #3972 from yiisoft/inflector-slug-improvements

Fixes #3939: `\yii\Inflector::slug()` improvements
Merge pull request #3878 from dapatrese/3877-twig-lexer-options-exception

Fixes Twig lexerOptions throwing exception (#3877)
Fixed changelog
Renamed `TwigSimpleFileLoader` into `FileLoader`
Fixed changelog wording
Merge pull request #3964 from klimov-paul/sphinx-gii

Enh: Sphinx Gii generator created