Fix crash when adding invalid documents to answers

We were rendering the `new` action, but that action doesn't exist.
Before commit ec861ca8e, we were rendering the `edit` action of an
answer, which was confusing as well.

Note that, when adding an invalid document, `@answer.documents` contains
that invalid document (which is not present in the database). Since
we're rendering the index, this new document would appear in the list of
the documents that can be deleted; to avoid that, we're kind of
"reloading" the answer object in the component by finding the record in
the database. We aren't using `@answer.reload` because doing so would
remove the validation errors.

app/controllers/admin/poll/questions/answers/documents_controller.rb

@@ -14,7 +14,7 @@ class Admin::Poll::Questions::Answers::DocumentsController < Admin::Poll::BaseCo
       redirect_to admin_answer_documents_path(@answer),
         notice: t("admin.documents.create.success_notice")
     else
-      render :new
+      render :index
     end
   end