The `use_helpers` method was added in ViewComponent 3.8.0, and it's
included by default in all components since version 3.11.0.
Note we sometimes delegated the `can?` method to the controller instead
of the helpers, for no particularly reason. We're unifying that code as
well.
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.
