Clean up attachable in attachments controller

- Access it via lazy loading #attachable method, exposed to the view as
  a helper method, rather than a find_attachable before filter.
- Find the attachable class from the params rather than using a case
  statement. This cleans up the controller and should make it simpler to
  reuse it for future attachables.
Remove unnecessary assignment in view
Clean up attachments controller attachment method

The views already use the attachment helper method, so this just cleans
up the controller by removing the before filter and making it clearer
that the attachment can be found or built, depending on params.
Tidy edition perm checks in attachments controller

Ideally the permissions system would be able to handle objects other
than editions so that can?(:see, attachable) would work for any type of
attachable. Given that it doesn't, and fixing the permissions system is
a bigger job, this tidies up the before filters so we only have a single
one related to edition checks instead of four.
Merge pull request #1171 from alphagov/attachment-controller-polymorphism

Tidy up admin attachments controller