Fix bug in bin/output-year-index - bad links

The script set the URL for the tarball of all entries in the year to:
$YYYY/$YYYY.tar.bz2 but the link is relative so for 1984, for example,
it would link to 1984/1984/1984.tar.bz2 which obviously is a 404.

I also changed the formatting and wording of the link. It now is all
bold and it says instead (for example with 1984): 'Download all winning
entries from 1984'. Before it still referred to 'entry source' but it is
all entries and more than just the source.

This commit does not mean that the YYYY/index.html files have been
checked (though it's likely that all should be okay at this point) but
the manifest of entries and the tarball of all entries of the year
should at least be good.

2005/index.html

@@ -471,7 +471,7 @@ See also <a href="../faq.html">the IOCCC FAQ</a> for additional information on t
 <h1 id="winning-entries-of-2005---the-18th-ioccc">Winning Entries of 2005 - The 18th IOCCC</h1>
 </div>
 </div>
-<p><strong>Download</strong> all <a href="2005/2005.tar.bz2">2005 IOCCC entry source</a></p>
+<p><a href="2005.tar.bz2"><strong>Download all winning entries from 2005</strong></a></p>
 <ul>
 <li><a href="aidan/index.html">2005/aidan</a> - Most ingenious puzzle solution</li>
 <li><a href="anon/index.html">2005/anon</a> - Best 3D puzzle</li>