Allow users to tweak the theme in /user.css

This was simultaneously easier, and harder than I anticipated ;-)

Originally I thought I'd just add a new css link in editor.html, but it turns out that has no way to access the configfs (I think). The real solution was harder than that...

But then I thought it was going to be a *lot* of javascript to make that connection, but as it turns out I just piggybacked on the existing theme-loading code, and it ended up being easier than I thought ;-)