Prep for separate consultation feedback & outcome

This is the first step towards adding support for adding public
feedback to consultations. This commit makes Response the base class
for ConsultationOutcome and updates the UI/controller code so that
we are now dealing with outcomes, not responses. The next step will
be to introduce the ConsultationFeedback model, which will be managed
in much the same way as outcomes.
Combine show and new page for responses

Also adds some contextual help on the new form.
Add ConsultationPublicFeedback to Consultations

We re-use the Responses controller and views to manage public feedback
on a separate tab. Some minor hoop jumping is required to make the
url routing work with singular resources.
ConsultationOutcome attachments support references
Correct typo
Change wording of hints on consultation forms

After discussion with Alice, we think this wording is more succinct. The
key concern is to alert people to the fact that they can upload
attachments on the next page, so a shorter message gives more chance
that they'll read this info.
ConsultationOutcome can have HOC fields
Add missing newline
Merge pull request #713 from alphagov/feedback-and-outcome

Consultations can now have "public feedback" and an "outcome"

https://www.pivotaltracker.com/story/show/50344699