Add modules and tabs logic to dashboard view

Now deals correctly with single module and tab dashboards requested by spotlight
This stuff feels like it should be in the same place as the [`spotlightify`](https://github.com/alphagov/stagecraft/blob/master/stagecraft/apps/dashboards/models/dashboard.py#L130) method. I'm not convinced that is here or in the model. I think we're getting enough logic in this to warrant a separate serialisation module. Discuss.