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.
We were using the same logic in four different places, so we're creating
a new class to handle that logic.
Note that I didn't find a way to delegate the `content` method to a
`Admin::TableActionsComponent`, so we're delegating the `action` method
instead. That means we need to create a method returning an
`Admin::TableActionsComponent`. We also need to cache this object;
otherwise we were getting an error when calling `actions.action` from
the `Admin::Poll::Questions::TableActionsComponent`.
In some cases (e.g. after editing or creating a resource
with errors) the default back_link did not redirect to the
expected page.
Now we force the back_links to the index pages, so we
always get the desired redirect.
