Commercial fluid 250 template

This ties up with the template in DFP, (code below) and pulls in the images and options from DFP to generate the fluid 250 style advert.

Guidelines for use of the template is here;
https://docs.google.com/a/guardian.co.uk/document/d/1n7Si4ZmJu9rQ5OxgXm7nCwVO6LbnrkabNi5ErpytC4k/edit?usp=sharing

DFP code;
```
<script type="application/json" class="breakout__script">
{
    "name": "fluid250",
    "params": {
        "link": "%%CLICK_URL_ESC%%[%Link%]",
        "trackingPixel": "[%Trackingpixel%]",
        "backgroundColor": "[%Backgroundcolour%]",
        "backgroundImage": "[%BackgroundImage%]",
        "backgroundRepeat": "[%Backgroundrepeat%]",
        "backgroundPosition": "[%Backgroundposition%]",
        "layerOneBGImage": "[%Layer1backgroundimage%]",
        "layerOneBGPosition": "[%Layer1backgroundimageposition%]",
        "layerTwoBGImage": "[%Layer2backgroundimage%]",
        "layerTwoBGPosition": "[%Layer2backgroundimageposition%]",
        "layerThreeBGImage": "[%Layer3backgroundimage%]",
        "layerThreeBGPosition": "[%Layer3backgroundimageposition%]",
        "layerOneBGImageM": "[%Layer1backgroundimagemobile%]",
        "layerOneBGPositionM": "[%Layer1backgroundimagepositionmobile%]",
        "layerTwoBGImageM": "[%Layer2backgroundimagemobile%]",
        "layerTwoBGPositionM": "[%Layer2backgroundimagepositionmobile%]",
        "layerThreeBGImageM": "[%Layer3backgroundimagemobile%]",
        "layerThreeBGPositionM": "[%Layer3backgroundimagepositionmobile%]"
    }
}
</script>
```
maybe make this more general so it can be used by other things - `creative__tracking-pixel`