Overview

On WriteFreely, allow user to switch between editors like we do on Write.as.

Mentioned in #671.

Other considerations

Note that some specialty instances might want to lock their users into one editor, e.g. classic, so they don't get confused. We should still allow for this.

One option would be to make this switchable state a new editor type. But we might still want to define a default editor (plain or rich), so consider that too.