Currently there is no AppFramework way to modify cookies, which makes it unusable for quite some use-cases or results in untestable code.
This PR adds some basic functionalities to add and invalidate cookies.
Usage:
```php
$response = new TemplateResponse(...);
$response->addCookie('foo', 'bar');
$response->invalidateCookie('foo');
$response->addCookie('bar', 'foo', new \DateTime('2015-01-01 00:00'));
```
Existing cookies can be accessed with the AppFramework using `$this->request->getCookie($name)`.
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