feat(media): add external media library support, Uploadcare integration (#1602)

dev-test/index.html

@@ -83,12 +83,10 @@
     var PostPreview = createClass({
       render: function() {
         var entry = this.props.entry;
-        var image = entry.getIn(['data', 'image']);
-        var bg = image && this.props.getAsset(image);
         return h('div', {},
           h('div', {className: "cover"},
             h('h1', {}, entry.getIn(['data', 'title'])),
-            bg ? h('img', {src: bg.toString()}) : null
+            this.props.widgetFor('image'),
           ),
           h('p', {},
             h('small', {}, "Written " + entry.getIn(['data', 'date']))