Custom Field Types setZone for getAssets

Following the documentation [here](https://docs.bolt.cm/3.0/extensions/advanced/customfields) instructions are given for registering custom assets.

However, this will not work unless the assets are assigned to a zone:

``` php
$asset = new JavaScript('web/jquery.uix.multiselect.min.js');
$asset->setZone(Zone::BACKEND);
return [
$asset
];
```