Fix comment notifications on legislation proposals

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.

spec/mailers/mailer_spec.rb

@@ -22,5 +22,11 @@ describe Mailer do
 
       expect(email).to deliver_from "New organization <new@consul.dev>"
     end
+
+    it "sends emails for comments on legislation proposals" do
+      email = Mailer.comment(create(:legislation_proposal_comment))
+
+      expect(email.subject).to include("commented on your proposal")
+    end
   end
 end