Fix removing existing image and adding a new one

We introduced a bug in commit acbd1b023.

When editing a record and removing an existing image, we don't remove
the HTML fields associated with that image but simply hide them, and
then we add fields to create a new image when clicking on "Add image".

This is standard cocoon behavior. However, since in the case of images
there's a `has_one` relation, cocoon doesn't add unique identifiers to
the new fields, generating duplicate IDs, which is invalid HTML.

Since there's a duplicate file input ID, clicking on the "Choose image"
label we aren't clicking on the new input but on the old one. This means
we aren't correctly attaching an image. The tests passed because
Capybara uses the equivalent of a keyboard to select the field, and in
this case everything worked properly.

So we need to delete the existing elements before inserting new ones.
We're adding a test to check there aren't duplicate IDs.

app/assets/javascripts/imageable.js

@@ -8,6 +8,9 @@
       $("#nested-image").on("cocoon:after-remove", function() {
         $("#new_image_link").removeClass("hide");
       });
+      $("#nested-image").on("cocoon:before-insert", function() {
+        $(".js-image-attachment").closest(".image").remove();
+      });
       $("#nested-image").on("cocoon:after-insert", function(e, nested_image) {
         var input;
         $("#new_image_link").addClass("hide");