Implemented meta twig helper

__Tasks:__

- [ ] gather feedback for my changes
- [ ] submit changes to the documentation

Related PR: https://github.com/sulu-cmf/sulu-standard/pull/374

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | none
| BC Breaks     | none
| Doc           | TODO
I guess this is ok, but seems a bit strange juggling the type of the $defaultLocale variable. If would do something more literal, e.g.

````php

$localization = $requestanalyzer->getCurrentPortal()->getDefaultLocalization();
$defaultLocale = $localization ? $localization->getLocalization() : null;
````