Simplify rendering document/image fields

We were adding <div> tags with the `images` or `documents` HTML class
prettly much every time we rendered a NestedComponent. We're now
including the HTML class inside the component, as we usually do.

We're also rendering the nested components directly, since it's been a
while since the partials were changed to simply render the components.

app/components/documents/nested_component.html.erb

@@ -1,4 +1,4 @@
-<fieldset class="documents-list">
+<fieldset class="documents-list documents">
   <legend><%= t("documents.form.title") %></legend>
   <p class="help-text"><%= note %></p>