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#: changelog</title>
</head>
<body>
<div class="buttons">
<a href="?cmd=tags">tags</a>
<a href="?cmd=manifest;manifest=#manifest#;path=/">manifest</a>
</div>
<h2>changelog for #repo#</h2>
<form>
search:
<input type="hidden" name="cmd" value="changelog">
<input name="rev" type="text" width="30">
navigate: #changenav#<br/>
</form>
#entries#
<form>
search:
<input type="hidden" name="cmd" value="changelog">
<input name="rev" type="text" width="30">
navigate: #changenav#<br/>
</form>
#footer#