Classification user groups individual stats

[yuenmichelle1]

Continuation of #24

What this PR introduces

• Logic for /classifications/user_groups/:id for ?individual_stats_breakdown=true param
• Slight change in user group project_contributions for response to include session_time along with project_id and count
• Slight change in user group top_contributors for response to include session_time along with user_id and count

Designs found here:

https://www.figma.com/file/qbqbmR3t5XV6eKcpuRj7mG/Group-Stats?type=design&node-id=0-1&mode=design

More details can be found in API docs here:

https://github.com/zooniverse/eras/wiki/API-Callout-Examples#classificationsuser_groupsid

(If don't feel like clicking above ^) here are example responses:

GET /classifications/user_groups/1234?individual_stats_breakdown=true

{
  "group_member_stats_breakdown": [
    {
      "user_id": 1234,
      "count": 12,
      "session_time": 71.304,
      "project_contributions": [
        {
          "project_id": 593,
          "count": 12,
          "session_time": 71.304
        }
      ]
    }
  ]
}

GET /classifications/user_groups/1234?top_contributors=1

{
  "total_count": 12,
  "time_spent": 71.304,
  "active_users": 1,
  "project_contributions": [
    {
      "project_id": 593,
      "count": 12,
      "session_time": 71.304
    }
  ],
  "top_contributors": [
    {
      "user_id": 1234,
      "count": 12,
      "session_time": 71
    }
  ]
}

[zwolf]

LGTM!