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.

test/functional/wiki_pages_controller_test.rb

@@ -23,6 +23,12 @@ class WikiPagesControllerTest < ActionDispatch::IntegrationTest
         assert_response :success
       end
 
+      should "render for a sitemap" do
+        get wiki_pages_path(format: :sitemap)
+        assert_response :success
+        assert_equal(WikiPage.count, response.parsed_body.css("urlset url loc").size)
+      end
+
       should "redirect the legacy title param to the show page" do
         get wiki_pages_path(title: "tagme")
         assert_redirected_to wiki_pages_path(search: { title_normalize: "tagme" }, redirect: true)