Delay starting the column grid until the feature editor is completely started

When laboratory would start and the user enabled the Tree mixin, the "First Name" column definition would not have `renderExpando: true` applied to it.  Adding a new column would cause everything to work. 

`laboratory/widgets/configForms/Tree` subscribes to the `/store/columns/update` topic and tracks the available columns.  The problem was the tree config-form was being loaded after the initial columns were added to the `ColumnGrid` and so it had not yet subscribed to `/store/columns/update` when the "First Name" and "Last Name" column definitions where added.

I modified `FeatureEditor` so the feature config file is now loaded in startup.  Because the config-form modules are loaded asynchronously, I made `FeatureEditor#startup` return a promise.  Now `Laboratory` does not start the `ColumnGrid` or add the initial column definitions until the `FeatureEditor#startup` promise is resolved.