handle user answers in server and mongo

[yishain11]

🧱 blocked: #17 #16 DOD:

• [ ] create a new endpoint in server for handeling answer from the user for a specific question
• [ ] in the req body send the user id, quesion id and answer
• [ ] update user stats for this answer, topic and difficulty in mongo (for example - if answered correctly a python easy question, update my personal stats for that)
• [ ] error handeling
• [ ] unit tests