Implement traffic light feedback

[lkeegan]

We have the required data now (see #54) to calculate the traffic light feedback for each milestone group:

• for each milestone group
  • for age range (child age +/- e.g. 6m) calculate s_bar + sigma of scores
  • calculate average score of child s
  • green if s > s_bar - sigma
  • yellow if s_bar - 2sigma < s < s_bar - sigma
  • red if s < s_bar - 2sigma

The details (how many sigma, what age range) may change but the overall scheme is now clear.

Should add logic to do this to the backend, plus endpoint to return a set of these traffic light feedbacks, one for each completed MilestoneAnswerSession, to be displayed in the frontend