Editor: Draft mode

[goldpbear]

We'd like to automatically save working editor changes client-side, either with a cookie or localstorage.

The longer-term, robust solution will be to automatically save changes made in editor mode to the backend behind the scenes. This might require new API endpoints or database structure. In the meantime, though, a localstorage solution will at least allow users to recover in-progress editor changes should they accidentally navigate away from an editor view.