extracted date format converting to a helper class

adding and reworking the conversion of:
- ICU to PHP
- PHP to ICU
- ICU to JUI
- PHP to JUI
Added tests for empty values

use "just now" for empty time difference in Formatter.

fixed wrong language name definition
Refactored DateValidator to support ICU format
Merge pull request #5027 from yiisoft/date-format-convert

[WIP] extracted date format converting to a helper class
typo
Fixed `yii\console\controllers\CacheController` does not check if cache component instance of 'yii\caching\Cache'

close #5055
avoid publishing dev files of js packages
use mb_string to work with non latin characters
fixed test break
Update structure-views.md

registerMetaTag() usage clarification
Merge pull request #5085 from tebazil/patch-4

Update structure-views.md
test case for #5084
Update structure-views.md

- We do not generate static pages in controllers, but merely output them.
- We need to say that the data is needed to be plainly put into the view, as the reader might expect something more sophisticated to happen here.
Merge pull request #5086 from tebazil/patch-5

[ci skip] Update structure-views.md
Russian translation of structure-views.md

Phew :)
Update structure-views.md

Formatting fix
Merge pull request #5087 from tebazil/master

[skip ci] Russian translation of structure-views.md
Improvements for debug toolbar (#838) - add panel to display used asset bundles
Fixes #5089: Added asset debugger panel
Fixes #5079: added test case.