[Gimp-web] Cleanup of admin/gimp-web-urls

[Raphaël Quinet]

I have just committed a change that has cleaned up a large part of
admin/gimp-web-urls.  That file is now less than half of its previous
size.  Basically, I have removed all substitutions that were never
used (there were lots of them) as well as most of those that were used
only once and were very unlikely to be used anywhere else.

This should simplify the maintenance of the list of books or the list
of mailing lists.  Several other files should also be easier to
maintain, especially for the cases that looked like this:
  <a href="wgo:some_description">http://example.org/</a>
Using a substitution like wgo:some_description when the URL was anyway
mentioned explicitly in the text created more work than necessary and
could lead to errors.

It should be possible to clean this up even more, but I hope that the
changes that I just committed will already simplify the maintenance of
the site.

I also hope that I did not break anything.  I wrote a script that
checks where each substitution is (or was) used so I think that
everything should be fine now.  But if you see some broken links,
please report them.

-Raphaël