Fix html markdown for span class="not-set" when time is not set

Translation review readme.md

See #4000

Update from remote

Create caching-overview.md

Add translate for caching-overview.md

Added more unit tests to widgets\Breadcrumbs and increased coverage to 100%

Update ActiveForm.php

Incorrect character for generating docs. See http://www.yiiframework.com/doc-2.0/yii-bootstrap-activeform.html

add guide-ru/rest-quick-start.md

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