[skip ci][UK][Guide] intro

Remove double "the"

Allow single items to choose their encoding option.

[skip ci] Translation of docs to russian (structure-widgets section)

Added Security::generateRandomHexKey(), used it for various tokens and default key generation

Fixes #4114: Added Security::generateRandomHexKey(), used it for various tokens and default key generation

fileinfo is not a hard dependency.

[skip-ci] Advanced app `User` model code style fixes

\yii\rbac\PhpManager adjustments

fixes #4086

changedAttributes of AfterSaveEvent contain an old values

Fixed a call of function "generateRandomKey()" in app\base\Security

Update BaseMessage.php

See https://github.com/yiisoft/yii2/commit/0ff0f9d32eb71cc13b7c6eea45f85d115f98a633

https://github.com/yiisoft/yii2/issues/4071

Conflicts:
	framework/UPGRADE.md

- Data is now stored in three separate files for items, assignments and rules. File format is simpler.
- Removed `authFile`. Added `itemsFile`, `assignmentsFile` and `rulesFile`.
- `createdAt` and `updatedAt` are now properly filled with corresponding file modification time.
- `save()` and `load()` are now protected instead of public.
- Added unit test for saving and loading data.