The `commentable_url` method wasn't updated when we added legislation
proposals.
Back when we first created this method, we couldn't pass budget
investments or topics directly to `polymorphic_url` because they are
nested resources. That isn't the case since commit ff93f5a59, so now we
can simplify this method.
We're keeping the `commentable_url` method for now in order to keep
compatibility with custom changes that might use it, although this
method isn't consistent with the `commentable_path` method (which
receives a comment, and not a commentable), and so we might have to
revisit this code in the future.
735f99f877