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

- 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.

Replaced `clearAll()` and `clearAllAssignments()` in `yii\rbac\ManagerInterface` with `removeAll()`, `removeAllRoles()`, `removeAllPermissions()`, `removeAllRules()` and `removeAllAssignments()`

Re-implemented RBAC by following more closely to the original NIST RBAC model. Dropped `yii\rbac\PhpManager`.

issue #2852