Provide additional metadata for notebook on server

None of this is in-scope for our alpha launch, but the following notebook features on the server would be useful:

- _description_ -- having some additional metadata description about the notebook would be useful. I think the `meta` part of the jsmd could have taken care of that, but now that we've phased it out, it'd be nice to give a high-level description of the notebook.
- _tags_ -- As we make public notebooks more discoverable, having tags seems like an easy way to let users find similar notebooks.
- _image_ -- It is often worthwhile to have an image represent the notebook. A field like this does indeed make it easier to visually identify a notebook. We'd need a field like this to enable that old iodide.io gifs in our example notebooks.