Bug 863033: Three basket fixes

1) When language, country, or format is not yet set in ET, the database
   there will have NULL and ET will return None. Fix that to '' so
   we can use it later without worrying about whether it's None.
2) Only set CREATED_DATE_ in ET when we create a new user in ET, not
   when we create a new user in Basket.
3) When the user changes languages and subscribes to things at the same
   time, use the new language they chose when sending their welcome
   messages.
Merge pull request #58 from dpoirier/three-basket-fixes