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.

fixes issue #3989

1. Added proper handling and support of the symlinked directories.
2. Added $options parameter.

Fixes #4071: `mail` component renamed to `mailer`, `yii\log\EmailTarget::$mail` renamed to `yii\log\EmailTarget::$mailer`

Fixes #3568: When the primary query sets `asArray`, it is not respected by the `via` relational query

issue #746

fixes #422

fixed some issues with schema detection

fixes #3233

- Added protected `\yii\Inflector::transliterate()` that could be replaced with custom translit implementation.
- Added proper tests for both intl-based slug and PHP fallback.
- Removed character maps for non-latin languages.
- Improved overall slug results.
- Added note about the fact that intl is required for non-latin languages to requirements checker.