Set user data that was initially blank

* If a field (country, language, format) was blank in ET, we were not
  updating it with the value submitted on the /existing page. Fix our
  detection of what data changed so we should now set those field.
* While we're here, add support for sorting newsletters by an 'order'
  field if we're given one. That'll be added to basket soon, so this
  will make bedrock ready to take advantage.
Use itemgetter; never cache newsletter existing

* Use operator.itemgetter in sort instead of lambda, just a little
  cleaner.
* Add @never_cache to our newsletter existing and updated views.
Fix tests - @never_cache doesn't get along with mock responses
Don't need @never_cache on the updated newsletter view
Merge pull request #843 from dpoirier/set-blank-values

Set user data that was initially blank