Adding prompt area to non-CodeCells to indent content.

This is a reponse to the problem of having really long lines in Markdown cells, which makes the content difficult to read. Users want wide code cells, so we don't want to narrow everything. The solution here is to give a prompt area to the heading/md cells to narrow their content area slightly. The only problem is that this makes it more difficult to distinguish between output and md content that follows that output. The solve this, we are adding a narrow line between output and following md.

This is a continuation of #4155 that removes the other stuff that wasn't ready for review/merge. I am going to close #4155.