seo: fix canonical tags on post index and show page.

* Fix incorrect canonical tags. Before we were using
  `<meta name="canonical" content="...">`. This is wrong, it should have been
  `<link rel="canonical" href="...">`.

* Add a default canonical tag on all pages. Fixes Google treating the
  same content on different subdomains (safebooru, shima, kagamihara, etc)
  as duplicate content. Also fixes Google sometimes treating similar but
  distinct content on the same domain as duplicate content.

test/functional/posts_controller_test.rb

@@ -1,6 +1,10 @@
 require "test_helper"
 
 class PostsControllerTest < ActionDispatch::IntegrationTest
+  def assert_canonical_url_equals(expected)
+    assert_equal(expected, response.parsed_body.css("link[rel=canonical]").attribute("href").value)
+  end
+
   context "The posts controller" do
     setup do
       @user = travel_to(1.month.ago) {create(:user)}
@@ -10,11 +14,29 @@ class PostsControllerTest < ActionDispatch::IntegrationTest
     context "index action" do
       setup do
         mock_post_search_rankings(Date.today, [["1girl", 100], ["original", 50]])
+        create_list(:post, 2)
       end
 
-      should "render" do
-        get posts_path
-        assert_response :success
+      context "for an empty search" do
+        should "render the first page" do
+          get root_path
+          assert_response :success
+          assert_canonical_url_equals(root_url(host: Danbooru.config.hostname))
+
+          get posts_path
+          assert_response :success
+          assert_canonical_url_equals(root_url(host: Danbooru.config.hostname))
+
+          get posts_path(page: 1)
+          assert_response :success
+          assert_canonical_url_equals(root_url(host: Danbooru.config.hostname))
+        end
+
+        should "render the second page" do
+          get posts_path(page: 2, limit: 1)
+          assert_response :success
+          assert_canonical_url_equals(posts_url(page: 2, host: Danbooru.config.hostname))
+        end
       end
 
       context "with a single tag search" do
@@ -22,6 +44,7 @@ class PostsControllerTest < ActionDispatch::IntegrationTest
           get posts_path, params: { tags: "does_not_exist" }
           assert_response :success
           assert_select "#show-excerpt-link", count: 0
+          assert_canonical_url_equals(posts_url(tags: "does_not_exist", host: Danbooru.config.hostname))
         end
 
         should "render for an artist tag" do
@@ -261,7 +284,7 @@ class PostsControllerTest < ActionDispatch::IntegrationTest
           get posts_path(format: :atom)
 
           assert_response :success
-          assert_select "entry", 1
+          assert_select "entry", 3
         end
 
         should "render with tags" do
@@ -272,7 +295,7 @@ class PostsControllerTest < ActionDispatch::IntegrationTest
         end
 
         should "hide restricted posts" do
-          @post.update(is_banned: true)
+          Post.update_all(is_banned: true)
           get posts_path(format: :atom)
 
           assert_response :success