Merge pull request #4084 from resurtm/4080-file-helper-remove-directory-symlinks

Fixes #4080, FileHelper::removeDirectory() symlinks handling enhancements
fixed rotateByCopy to avoid any rename()

fixes issue #3989
Update start-installation.md
'yii\base\Security' component created.
Component 'security' added tp the base application
Update start-looking-head.md
'Security' helper usage switched to 'security' application component.
Docs regarding `Security` component usage updated.
Security helper files removed
Notes about `Security` class refactoring added to CHANGELOG.md and UPGRADE.md
Doc comments at `yii\base\Security` fixed
Update start-workflow.md
Update structure-application-components.md
Update structure-applications.md
Update structure-controllers.md
changed default for rotateByCopy of FileTarget

works on windows out of the box now.
fixes #3989
Extra namespace at docs removed
Fixed typo. [skip ci]
Fix possibility of non-removed symlinked directory.
changedAttributes of AfterSaveEvent contain an old values