Clean up response factory

We shouldn't be using factories to build abstract responses, just the
concrete subclasses (consultation outcome and consultation public
feedback). If an abstract response really is wanted it can be built
using Response.new.

Also responses don't make sense without a consultation, so make sure we
build one in the factory.
More thorough attachments controller poly tests

Test view rendering for all the actions for all the expected attachable
types, since there was some functionality which didn't work for new
types.
Make attachments#index work for non-editions
Split attachment UI for policy advisory groups

Add tests for policy advisory groups to attachments controller, tabs to
the policy advisory groups form, and swap out the attachments fields for
the markdown reference.
Stop accepting nested attributes for attachments

Now that all attachment editing is done via the admin attachments
controller we no longer need to accept nested attributes. This removes
the nested attributes and all tests which was checking that behaviour.
Remove unused partial