Illustration URL uses the book UUID

If the server is initialized with a library.xml file, then the id
specified in the XML file is used (rather than the UUID recorded in the
ZIM file).

Note that in test/data/library.xml the book ids are fake and
different from the real ZIM IDs; that file was created for testing
of the /catalog endpoint which doesn't access ZIM content, so the
the same ZIM file zimfile.zim was added to library.xml three times as
three different books (with unique human-friendly ids). This explains
the diff in test/library_server.cpp.

static/templates/catalog_v2_entry.xml

@@ -16,7 +16,7 @@
     <articleCount>{{article_count}}</articleCount>
     <mediaCount>{{media_count}}</mediaCount>
     {{#icons}}<link rel="http://opds-spec.org/image/thumbnail"
-          href="{{root}}/catalog/v2/illustration/{{{content_id}}}/?size={{icon_size}}"
+          href="{{root}}/catalog/v2/illustration/{{{id}}}/?size={{icon_size}}"
           type="{{icon_mimetype}};width={{icon_size}};height={{icon_size}};scale=1"/>
     {{/icons}}<link type="text/html" href="{{root}}/{{{content_id}}}" />
     <author>