Fix references to Capybara.app_host
Fix references to Capybara.app_host

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.

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