Feature: hint
- data: find synonym for every 'targets' word in word-bank*.json, and add the synonym undert the key 'hint' for the word.
- server: when 5th guess is not all green, return in API additional key: "hint": SYNONYM" with the recorded synonym  
- client: when API returns "hint", display the text "hint: value_of_the_hint" below guessin grid. In Russian UI use translation for the word "hint".