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

fixes #3244

fixes #3245, #3244

fixes #3539

```
vendor/bin/indent --spaces -r framework/
vendor/bin/indent --tabs -r . --pattern=*.json
vendor/bin/indent --spaces -r . --pattern=*.md
vendor/bin/indent --spaces -r . --pattern=yii.*.js
vendor/bin/indent --spaces -r . --pattern=*.css
```

issue #3494

implementation checks for instanceof for additional swiftmailer
capabilities.

issue #2870

Fixes #1263: Fixed the issue that Gii and Debug modules might be affected by incompatible asset manager configuration

fix https://github.com/yiisoft/yii2/issues/3050

1xx status is not that common.

issue #3026

Fixes #2912: Relative view files will be looked for under the directory containing the view currently being rendered

fixes #2765