Use ng-src for img src

Using the [ng-src](https://docs.angularjs.org/api/ng/directive/ngSrc) directive instead of src attribute on images prevents the browser from loading images with the angular markup still in the URL, which leads to extraneous 404s e.g. in #298,
`http://127.0.0.1:8888/html/%7B%7Blisting.contract_body.Contract.item_images%7D%7D`