hgweb: add tags links and manifest links
This adds a simple new tags page to browse by tag, adds the tag link
everywhere, and adds manifest links to the tags and changelog pages.
#header#
<title>#repo#: manifest #manifest#</title>
</head>
<body>
<div class="buttons">
<a href="?cmd=changelog;rev=#rev#">changelog</a>
<a href="?cmd=tags">tags</a>
<a href="?cmd=changeset;node=#node#">changeset</a>
</div>
<h2>manifest: #path#</h2>
<div class="parity1"><a href="?cmd=manifest;manifest=#manifest#;path=#up#">[up]</a><br /></div>
#entries#
#footer#