Clean up Response model

Gets rid of a lot of the cruft that was required for dealing with
nested attributes. As part of this, Response published_on is now a
required attribute. The previous code did some crazy hoop jumping so
that the published_on date was set auto-magically if the response
had a summary or an attachment (because nested attributes lols).
Pre-fill response published date

To make things a little easier for editors, the published_on date is
preset to the current date when creating a new response. This also
tidies up the form (it's a date after all, not a date time...)