Fix admin navigation not rendering

Not sure why this started happening now, but the admin navigation
dropdown wasn't receiving its children properly. This commit fixes a
flaw in our Mithril patch and allows an array of children to be passed
in the normal JSX way, rather than as an attribute.

framework/core/js/admin/src/components/AdminNav.js

@@ -18,9 +18,9 @@ export default class AdminNav extends Component {
     return (
       <SelectDropdown
         className="AdminNav App-titleControl"
-        buttonClassName="Button"
-        children={this.items().toArray()}
-        />
+        buttonClassName="Button">
+        {this.items().toArray()}
+      </SelectDropdown>
     );
   }