Move Atom link to head.html

Previously, only paginated indices (/, /page/2/, etc) and tag pages
(/tag/lanyon/, etc) contained Atom links. Ideally, I'd rather have every
page containing an Atom link. Tag pages should contain the tag-specific
feed, but every other page should have the site-wide feed.

This commit moves the link to head.html (which is included in every
page) and ensures that the site-wide feed link doesn't clobber any tag-
specific feed links.