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

* highlight missing translations on a page
* 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 

@Demwunz could you also look at this one as I've added some styling to `_common.scss`. Not sure if this is the right place and we probably also wanna only do this in development and staging.