forum: optimize unread forum topics on forum index.

Avoid doing one SQL query per topic when checking for new topics on the
forum index.

This also changes it so that forum topics aren't always marked as new
for anonymous users.

test/unit/forum_topic_test.rb

@@ -15,7 +15,7 @@ class ForumTopicTest < ActiveSupport::TestCase
       CurrentUser.ip_addr = nil
     end
 
-    context "#read_by?" do
+    context "read_by_user" do
       context "with a populated @user.last_forum_read_at" do
         setup do
           @user.update_attribute(:last_forum_read_at, Time.now)
@@ -26,8 +26,8 @@ class ForumTopicTest < ActiveSupport::TestCase
             @topic.update_column(:updated_at, 1.day.from_now)
           end
 
-          should "return false" do
-            assert_equal(false, @topic.read_by?(@user))
+          should "return nothing" do
+            assert_equal([], ForumTopic.read_by_user(@user).map(&:id))
           end
         end
 
@@ -41,8 +41,8 @@ class ForumTopicTest < ActiveSupport::TestCase
               FactoryBot.create(:forum_topic_visit, user: @user, forum_topic: @topic, last_read_at: 16.hours.from_now)
             end
 
-            should "return false" do
-              assert_equal(false, @topic.read_by?(@user))
+            should "return nothing" do
+              assert_equal([], ForumTopic.read_by_user(@user).map(&:id))
             end
           end
 
@@ -51,8 +51,8 @@ class ForumTopicTest < ActiveSupport::TestCase
               FactoryBot.create(:forum_topic_visit, user: @user, forum_topic: @topic, last_read_at: 2.days.from_now)
             end
 
-            should "return true" do
-              assert_equal(true, @topic.read_by?(@user))
+            should "return the topic" do
+              assert_equal([@topic.id], ForumTopic.read_by_user(@user).map(&:id))
             end
           end
         end
@@ -60,8 +60,8 @@ class ForumTopicTest < ActiveSupport::TestCase
 
       context "with a blank @user.last_forum_read_at" do
         context "and no visits" do
-          should "return false" do
-            assert_equal(false, @topic.read_by?(@user))
+          should "return nothing" do
+            assert_equal([], ForumTopic.read_by_user(@user).map(&:id))
           end
         end
 
@@ -71,8 +71,8 @@ class ForumTopicTest < ActiveSupport::TestCase
               FactoryBot.create(:forum_topic_visit, user: @user, forum_topic: @topic, last_read_at: 1.day.ago)
             end
 
-            should "return false" do
-              assert_equal(false, @topic.read_by?(@user))
+            should "return nothing" do
+              assert_equal([], ForumTopic.read_by_user(@user).map(&:id))
             end
           end
 
@@ -81,8 +81,8 @@ class ForumTopicTest < ActiveSupport::TestCase
               FactoryBot.create(:forum_topic_visit, user: @user, forum_topic: @topic, last_read_at: 1.day.from_now)
             end
 
-            should "return true" do
-              assert_equal(true, @topic.read_by?(@user))
+            should "return nothing" do
+              assert_equal([], ForumTopic.read_by_user(@user).map(&:id))
             end
           end
         end