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.

@pmclanahan