Fixes #259 Reflect file deletion by removing the whole entry

Remove the whole file element from the exercise form when a file is
deleted.

Author: arkirchner

app/assets/javascripts/exercises.js

@@ -87,12 +87,12 @@ $(document).on('turbolinks:load', function () {
         // validate fileUrl
         const matches = fileUrl.match(/files\/(\d+)/);
         if (matches) {
-            // select the file form based on the delete button
-            const fileForm = $(`*[data-file-url="${fileUrl}"]`).parent().parent().parent();
-            fileForm.remove();
-
-            // now remove the hidden input representing the file
             const fileId = matches[1];
+
+            $(`#file-form-${fileId}`).remove();
+
+            // The remaining hidden input field associated with the
+            // deleted file needs to be removed.
             const input = $(`input[type="hidden"][value="${fileId}"]`)
             input.remove()
         }

app/views/exercises/_file_form.html.slim

@@ -1,6 +1,6 @@
 - id = f.object.id
 
-li.card.mt-2
+li.card.mt-2 id="file-form-#{id}"
   .card-header#heading role='tab'
     - collapsed_class = f.index == 'index' ? nil : 'collapsed'
     - aria_expanded = f.index == 'index' ? 'true' : 'false'