Fixes #4489: RBAC PhpManager wasn't saving multiple assignmentsFixes #4489: RBAC PhpManager wasn't saving multiple assignments properly

Security component adjustments: fixed comment style, hkdf() and pbkdf2() are now protected, compareString() is now public

Fixes #4453: `yii message/extract` wasn't properly writing to po files in case of multiple categories

Fixes #4318: `yii\helpers\Html::ul()` and `ol()` will return an empty list tag if an empty item array is given

Fixes #4051: Renamed `yii\caching\GroupDependency` to `TagDependency` and added support for associating multiple tags to a single cached data item

Adjusted Connection::transaction to provide Connection instance by default instead of transaction instance

do not use scripting as it is disabled by default and will cause edge
case queries to fail

fixes #4187

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

fixes #4086

- 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.