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#: #file# history</title>
</head>
<body>
<div class="buttons">
<a href="?cmd=changelog">changelog</a>
<a href="?cmd=tags">tags</a>
<a href="?cmd=file;file=#file#;filenode=#filenode#">file</a>
<a href="?cmd=annotate;file=#file#;filenode=#filenode#">annotate</a>
</div>
<h2>#file# revision history</h2>
#entries#
#footer#