Add create204Response method

## Add create204Response method

Minor "annoyance" optimization here. Whenever we make 204 responses, we always have to do this:

```PHP
return $this->createSimpleResponse(204, '');
```

But the specification says 204 responses should *always* have an empty response body. It's very slightly annoying to always have to specify it. It would be nice to just do:

```PHP
return $this->create204Response();
```

### Acceptance Criteria
1. `AbstractController::create204Response` method exists, and returns an HttpFoundation Response object with 204 as the status code and an empty string as the body.

### Tasks
- Create method