Fixes #3066: `yii\db\mssql\Schema::getTableSchema()` should return null when the table does not exist.

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Adjust code standard

issue #3026

fixes #3034

New addSelect method. Useful for scopes.

fixes #2650

Fixes #2955: Changed the signature of ActiveQuery constructors and `ActiveRecord::createQuery()` to simplify customizing ActiveQuery classes

fixes #2957

fix for https://github.com/yiisoft/yii2/pull/2958

Refactored ActiveRecord::find() so that find(null) will also return an ActiveRecord instance instead of ActiveQuery.

issue #2892

so information about changed values is available in `afterSave`-event.

fixes #2892