Collapse files in exercise edit view

app/views/exercises/_file_form.html.slim

@@ -1,4 +1,10 @@
+- id = f.object.id
 li.panel.panel-default
+  .panel-heading role="tab" id="heading"
+    div.clearfix role="button"
+      span = f.object.name
+      a.pull-right data-toggle="collapse" data-parent="#files" href="#collapse#{id}" collapse
+  .panel-collapse.collapse.in id="collapse#{id}" role="tabpanel"
     .panel-body
       .clearfix = link_to(t('shared.destroy'), '#', class:'btn btn-warning btn-sm discard-file pull-right')
       .form-group

app/views/exercises/_form.html.slim

@@ -29,7 +29,7 @@
       = f.check_box(:hide_file_tree)
       = t('activerecord.attributes.exercise.hide_file_tree')
   h2 = t('activerecord.attributes.exercise.files')
-  ul#files.list-unstyled
+  ul#files.list-unstyled.panel-group
     = f.fields_for :files do |files_form|
       = render('file_form', f: files_form)
   a#add-file.btn.btn-default.btn-sm.pull-right href='#' = t('.add_file')