Use Capybara to disable animations in tests

We were doing it manually, but Capybara offers an option which does the
exact same thing.

This way we also apply the NoJavascriptTagHelper ERB rule, which
reported one error in the `disable_animations_in_tests` partial.

app/views/layouts/_common_head.html.erb

@@ -6,4 +6,3 @@
 <%= javascript_include_tag "application", "data-turbolinks-track" => "reload" %>
 <%= csrf_meta_tags %>
 <%= favicon_link_tag "favicon.ico" %>
-<%= render "layouts/disable_animations_in_tests" if Rails.env.test? %>

app/views/layouts/_disable_animations_in_tests.html.erb

@@ -1,8 +0,0 @@
-<%= javascript_tag "jQuery.fx.off = true" %>
-<style>
-* {
-  transition: none !important;
-  animation-duration: 0s !important;
-  animation-delay: 0s !important;
-}
-</style>