Tag and category-specific ATOM feed links in header of respective web pages

This should make it much easier for people to get subset feeds, by simply pointing to the appropriate summary page.
Only base.html template is modified:
Include ALL feed links only when serving index.html.
Include tag-specific feed in {{ tag }}.html if TAG_FEED_ATOM is configured.
Include category-specific feed in {{ category }}.html if CATEGORY_FEED_ATOM is defined.