Fixes #3674: Various Twig enhancements:

- Removed `FileLoader` and used `\Twig_Loader_Filesystem` instead.
- Added support of Yii's aliases.
- Added `set()` that allows setting object properties.
[skip ci] Fixes #4548
Fixes Twig to ignore non-path aliases instead of erroring
Reduced gap between setting and checking mtime
Fixed tests to take deferred validation signature change into account
Added string check to Twig renderer alias auto adding
Relaxed access level for `yii\gii\generators\model\Generator`
Fixes #2315: Any operator now could be used with `yii\db\Query::->where()` operand format
Fixes #4556: Gii was incorrectly generating labels
Remove unnecessary Markdown code characters.
Fix Markdown link to the Yii class.
Update MessageController.php

Fixed order of checking config `format` property
Merge pull request #4561 from maxlapko/patch-1

Update MessageController.php
test and docs to be clear about #4558
Better alias handling for Twig extension
Minor template engine doc enhancement
Added support for arbitrary operators to Query::filterWhere, added support for arbitrary operators to Sphinx extension
Removed unnecessary code
Simplified parameter check for buildSimpleCondition()
Fixes #4425: generate cookie validation keys in local config files.