hgweb: recreate old DOM structure for css in monoblue style

There's a "p.changeset-age span" css block in style-monoblue.css with quite a
bit of rules, including position. They were all unused, since there weren't
matching span element inside the p.changeset-age.

The span was removed in b24e5a708fad (as it seemed meaningless at the time?)
and since then relative changeset age text looked weird and broken.

"age" class is used for calculating relative changeset age in javascript: all
content of such element is replaced with human-friendly text (e.g.
"yesterday"). So the new span gets the age class.

mercurial/templates/monoblue/changeset.tmpl

@@ -39,7 +39,7 b''
     <h2 class="no-link no-border">changeset</h2>
 
     <h3 class="changeset"><a href="{url|urlescape}raw-rev/{node|short}">{desc|strip|escape|firstline|nonempty} <span class="logtags">{inbranch%inbranchtag}{branches%branchtag}{tags%tagtag}{bookmarks%bookmarktag}</span></a></h3>
-    <p class="changeset-age age">{date|rfc822date}</p>
+    <p class="changeset-age"><span class="age">{date|rfc822date}</span></p>
 
     <dl class="overview">
         <dt>author</dt>

mercurial/templates/monoblue/fileannotate.tmpl

@@ -41,7 +41,7 b''
 
     <h2 class="no-link no-border">{file|escape}@{node|short} (annotated)</h2>
     <h3 class="changeset">{file|escape}</h3>
-    <p class="changeset-age age">{date|rfc822date}</p>
+    <p class="changeset-age"><span class="age">{date|rfc822date}</span></p>
 
     <dl class="overview">
         <dt>author</dt>

mercurial/templates/monoblue/filerevision.tmpl

@@ -41,7 +41,7 b''
 
     <h2 class="no-link no-border">{file|escape}@{node|short}</h2>
     <h3 class="changeset">{file|escape}</h3>
-    <p class="changeset-age age">{date|rfc822date}</p>
+    <p class="changeset-age"><span class="age">{date|rfc822date}</span></p>
 
     <dl class="overview">
         <dt>author</dt>