Refactor static function outputXML and outputJSON in API

[jeroendesloovere]

Problem

When replacing SPOON_CHARSET by $container->getParameter('kernel.charset'),
Issue: #1138
PR: #1140
We saw that we had to use BackendModel just for getting the container in a static function.

Solution from @WouterSioen

We can remove that use Backend\Core\Engine\Model as BackendModel again if we change these static functions to normal functions if we use $this->output in outputXML and outputJSON

[WouterSioen]

The static isAuthorized and isValidRequestMethod methods are kind of a deal breaker.

I'd try to put the api in the container, so you could do BackendModel::get('api')->isAuthorized(), instead off BaseApi::isAuthorized. (and the same for isValidRequestMethod).

This way, all methods that call self::output could be made non-static!

[carakas]

@jeroendesloovere is this still an issue or was his solved already

[jeroendesloovere]

@WouterSioen Can you take a look at this, since you are more aware of the issue.

[WouterSioen]

The API needs to be deprecated and altered by an easy way to add new API endpoints. This will also fix the usage of SpoonTemplate.