Remove CurrentUser.ip_addr.

Remove the `CurrentUser.ip_addr` global variable and replace it with
`request.remote_ip`. Before we had to track the current user's IP in a
global variable so that when we edited a post for example, we could pass
down the user's IP to the model and save it in the post_versions table.
Now that we now longer save IPs in version tables, we don't need a global
variable to get access to the current user's IP outside of controllers.

test/unit/forum_topic_test.rb

@@ -6,13 +6,11 @@ class ForumTopicTest < ActiveSupport::TestCase
       travel_to Time.now
       @user = FactoryBot.create(:user)
       CurrentUser.user = @user
-      CurrentUser.ip_addr = "127.0.0.1"
       @topic = create(:forum_topic, title: "xxx", creator: @user)
     end
 
     teardown do
       CurrentUser.user = nil
-      CurrentUser.ip_addr = nil
     end
 
     context "#mark_as_read!" do