Improve the tag page and fix the Tag RSS feeds

Improvements to bring:

• Display a message if there are not tags
• Fix the link to the Rss feed. It should be TagsRss and not TagRss
• Improve the TagRss page so that when getting the feed for all tags, do not include documents with empty tags
• Add the Rss icon to the page listing all pages for a given tag
• Add the Rss icon the main page for getting a feed for all tags
• Add the Tag feeds to the RssFeeds page (see XE-78)