Replace Slate with ProseMirror
This replaces the Slate rich-text editor in xpub-edit
with ProseMirror.
The main advantages are:
- A stable API, now that ProseMirror 1.0 has been released.
- A solid editor.
- Easy to define and enforce a schema.
- Simpler menu, as the core handles more of the actions.
- Better DOM parsing and serializing.