show Assemble Blocks Here: message even if we don't show block counts

instead of setting the entire workspace-header to 'hidden', we now do it only to a 'blockCounter' div.

The 'blockCounter' id was previously assigned to the span with that message, but the id was not referenced. So I moved that id to a new div that wraps the elements that actually show the counters.

I kept the msg.workspaceHeader() string in a span because the existing styles expected it to be in a span.