[racket-dev] Fix for PR13305 displays oddly in Opera

Ok, I'm reverting most of what I did here for PR 13305, but I do
introduce a change in the sequence:

    <span class="mywbr"> &nbsp;</span>

where the discretionary word breaks are supposed to happen: the &nbsp;
is what I've adding.  The extra &nbsp; entity is bizarre, but I've
seen it recommended elsewhere on the web when wbr is being simulated
via css tricks.  Adding it corrects the peculiar layout issue that
Eric was seeing with the display of _gcpointer.  I've used
browsershots.org to observe that this is compatible with the major
browsers.