Just like we did with regular attachments, we're moving the logic to
generate URLs out of the model.
Note we're changing the `image_path_for` helper method in order to
return a `polymorphic_path` because sometimes it's used in combination
with `favicon_link_tag`, and `favicon_link_tag` doesn't automatically
generate a polymorphic URL when given an `ActiveStorage::Attachment`
record.
