highlight missing translations, use relative keys, remove "copy" key, use translations in features

* Raise exceptions in dev and test when translations are missing
* use relative translation keys where possible (moved some keys around to 
  facilitate this including removal of 'copy' key)
* move some keys for claim_reviews into `claim_review.en.yml`
* start to use translation helpers in features 

Frigged about trying to log exceptions in production but couldn't get this to work:
http://guides.rubyonrails.org/i18n.html#using-different-exception-handlers

So raising is just done in dev and test for now. Should cover most cases.