in migration controller to be compatible to more backends

fixes #3947

fixes #4123

Schema name was not used in ’SHOW CREATE TABLE’  query

Fixed bug where schema name was not used in ’SHOW CREATE TABLE’ query in `yii\db\mysql\Schema`

close #4162

translate exception message

Fixes #4147

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

fileinfo is not a hard dependency.

fixes #4086

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

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

- 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.