Add new guide upload-file

1.  UploadedFile::getInstances($model, 'file'); sees only one file if not set ```'file[]'```

```php
<?= $form->field($model, 'file[]')->fileInput(['multiple' => '']) ?>
```

2. ```$form->errorSummary(..)``` not return all errors of file attribute, but only the last.
So I had to invent.

```php
if ($model->hasErrors()) {
    echo '<pre>';
    print_r($model->getErrors());
    echo '</pre>';
}
```

So it should be?
Update DefaultValueValidator.php

Allow the default value to be a closure
Update CHANGELOG.md
Update DefaultValueValidator.php
MS SQL Server fix for older versions than 2012 (alternative solution)

This alternative solution LIMIT, OFFSET problem SQL Server version older than 2012.
Check if property is set

```getValue``` didn't check if the property on an object exists so wouldn't return the default value.
fix error

fix error when if fields whose names are reserved word
Update QueryBuilder.php

fix error occurs when you use multiple databases.
fix #3478

it correct work with integer keys of list
Update QueryBuilder.php

solution used **heukirne** for the names of columns.
Method to unlink all relationship many to many in current model
add change to CHANGELOG,and mini refactoring
add #3520 to changelog
change nik name (case sensitive)
Add support all type relation
limit applied to migration
limit condition changed
Mail tutorial created
Mail tutorial finished
Merge branch 'master' of github.com:yiisoft/yii2