allow bundled widgets to be hidden from dropdown, and nested (#714)

This makes a few refinements to bundling widgets:

  * A widget with `published: false` is not shown in the
    custom widget dropdown in the UI. This is so widgets
    can be bundled with the app for "native" use (like the
    calendar widget) without immediately resulting in an
    extra listing in the UI. (There are improvements we'd
    like to make to the UI to better communicate widget
    provenance and quality eventually, which would be a
    helpful alternative to just a binary flag.)

  * A relative path to the custom widget manifest is
    respected. This will make the bundling process marginally
    neater.

app/common/CustomWidget.ts

@@ -31,6 +31,11 @@ export interface ICustomWidget {
    */
   renderAfterReady?: boolean;
 
+  /**
+   * If set to false, do not offer to user in UI.
+   */
+  published?: boolean;
+
   /**
    * If the widget came from a plugin, we track that here.
    */