Don't include unneeded helpers in tests

Including them might lead to conflicts since two methods might have the
same name. For example, we're getting some exceptions when taking
screenshots of a failing test, because the method `image_path` from
`ActionView::Helpers::AssetUrlHelper` has the same name as a method used
to save the screenshot.

Besides, we were including all helpers in places were only the `dom_id`
method is used, and in other places where no helper methods were used at
all. So we can just invoke `ActionView::RecordIdentifier.dom_id`
directly.

spec/shared/models/image_validations.rb

@@ -1,5 +1,4 @@
 shared_examples "image validations" do |imageable_factory|
-  include ImagesHelper
   include ImageablesHelper
 
   let!(:image)                  { build(:image, imageable_factory.to_sym) }