seo: increase sitemap coverage.

Rework sitemaps to provide more coverage of the site. We want every
important page on the site - including every post, tag, and wiki page -
to be indexed by Google. We do this by generating sitemaps and sitemap
indexes that contain links to every important page on the site.

app/views/application/index.sitemap.erb

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="UTF-8"?>
+
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+  <% @current_item.each do |item| %>
+    <url>
+      <loc><%= polymorphic_url(item) %></loc>
+      <lastmod><%= item.updated_at.iso8601 %></lastmod>
+    </url>
+  <% end %>
+</urlset>