Implement endpoint for contest group scoreboard

[fushar]

See also: #423

Some details:

• Only Uriel admins can hit the endpoints.
• Implementations should be similar to ContestScoreboardService.
• The contract, store, and model layers have been partially implemented.

1. Get contest group scoreboard

   • Path: GET /api/v2/contest-groups/{contestGroupJid}/scoreboard?frozen=xxx

   • Response:

     ContestGroupScoreboardResponse:
     - ContestScoreboard data
     - Map<String, Profile> profilesMap