Pull out consultation response to separate tab

This implements the basic CRUD for adding and editing consultation
responses outside of the main edition form
Add UI for managing response attachments

This makes Admin::AttachmentsController polymorphic such that it can
be used to manage attachments on responses as well as editions. In
theory, very little additional work will be required to use it
for managing attachments on the remaining "attachable" models in the
system.

This has been complicated slightly by the fact that Response does
not use the Attachable module and has it's association and join model
for attachments set up slightly differently than the rest. For now,
I've simply added the relevant methods directly to the Response class
and adjusted the relevant association accordingly. Hopefully, a later
commit will DRY things up and make Response "Attachable", assuming it
doesn't have any strange behaviour that stops me unifying things (for
a start, the join model naming does not conform to that of the
Attachable module...)
Generate correct tabs for the attachable thing

Consultations get a different set of tabs to other editions. This
tabbing code is about ready for a refactor...
Clean up old response handling code

We have to leave the crufty code for handling response forms in place
for now. Still, look at all the red!