Using `render :nothing` was deprecated, but we never noticed it because
we didn't have a test for the action using it. In Rails 5.1, it raises
an exception.
Using `head :ok` and adding a test for this scenario solves the issue.
CKEditor repositions the dialog to attach an image after showing it. If
its position changes right when Capybara is trying to click the "Upload"
link, the click does not work properly.
I haven't found a way to check the dialog has stopped moving, so I've
chosen to force its position to the top.
Without this change, this test failed a lot on my machine when using
Rails 5.1, both with an "iframe ckeditor" and an "inline ckeditor".
However, it didn't fail when using Rails 5.0.
We were manually setting `http://www.example.com`. However, Capybara now
uses `http://127.0.0.1`.
While we could change the code to use `127.0.0.1`, I think directly
using `Capybara.app_host` makes it easier to realize what the code is
doing. And, particularly, now it's clear the host has nothing to do with
our `Setting["url"]`, which by default points to `www.example.com` as
well.
