I think we need to keep the `BLOCKING_ELEMENT_SPACER_CLASS` as a separate thing, but those elements should also have the `EDITOR_ONLY_CLASS`. It's more explicit to say that this is a blocking element spacer AND it needs to be removed, rather than just using it to space blocking elements. It makes it obvious to anyone looking at the DOM why those little BR's keep popping up.

Also, without two classes, this selector might have some problems in the future. If we had to add another kind of `br` for a different purpose, we wouldn't be able to distinguish between them.