Extra fields for commercial expandable template

This adds tracking pixel and url fields for the expandable DFP template.

DFP template output as below;

```
<script type="application/json" class="breakout__script">
    {
        "name": "expandable",
        "params": {
            "slide1": "[%Slide1%]",
            "slide2": "[%Slide2%]",
            "cta": "[%CTA%]",
            "logo": "[%Logo%]",
            "text": "[%Text%]",
            "textMobile": "[%TextMobile%]",
            "clickMacro": "%%CLICK_URL_ESC%%",
            "trackingPixel": "[%Trackingpixel%]%%CACHEBUSTER%%",
            "link": "%%CLICK_URL_ESC%%[%Link%]"
        }
    }
</script>
```
It's added at the DFP level rather than here.