Format the old and new dates consistently on the edit page

- Format as 'Day Month Year' in its long form, i.e. 11 September 2014,
  in GOV.UK style.
- Use the date picker field Rails helper I found about about from
  Whitehall to make it easier for people to tell the difference between
  months and days - it displays as three separate dropdowns:
  `2014 September 11` for example. The clunky (to me) `Date.new` to get
  it into the database as yyyy-mm-dd is due to `date_select` returning a
  params hash like:
  `{"launch_date(1i)"=>"2014", "launch_date(2i)"=>"11", "launch_date(3i)"=>"11"}`,
  and I found the solution used on http://stackoverflow.com/a/23353602.