Make initTwigEnvironment public

I'm trying to render a cms Partial (and soon Content too) from a random method and am using the following code
```php
$theme = Theme::getEditTheme();
$controller = new Controller($theme);
$controller->initTwigEnvironment();

return $controller->renderPartial('foo');
```

Unfortunately this isn't currently possible due to `initTwigEnvironment()` being protected. This pull request fixes that.

Unless there is a better way?