Refactor.
Now word-bank*.json files have 4 elements in 'targets' object: pre-defined 'id', 'word', and calculated 'difficulty' and 'replays'. 
The 'difficulty' and 'replays' are returned by API on the last guess.
Can 'difficulty' and 'replays' be calculated at runtime before returned by API, and the two fields removed from word-bank*.json files?   
