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/functional/comments_controller_test.rb

@@ -8,12 +8,10 @@ class CommentsControllerTest < ActionDispatch::IntegrationTest
       @post = create(:post, id: 100)
 
       CurrentUser.user = @user
-      CurrentUser.ip_addr = "127.0.0.1"
     end
 
     teardown do
       CurrentUser.user = nil
-      CurrentUser.ip_addr = nil
     end
 
     context "index action" do