Prevent back button for adding score multiple times using back button

[anisfeld]

Describe Your Changes

• add logic to prevent scores being add multiple times for a single round
• updated database with game_player.completed_game flag
• adjust the all_set function to protect against students answering questions while other students are still seeing waiting ... thereby breaking previous logic and getting everyone stuck.
• add some exception handler wrappers

Non-Obvious Technical Information

Checklist Before Requesting a Review

• [x] The code runs successfully.

% curl localhost:8000/games/current_score/21
% 0                                                                       
% curl -X PUT localhost:8000/games/4/player/21/update_score
% curl localhost:8000/games/current_score/21
% 100
% curl -X PUT localhost:8000/games/4/player/21/update_score
% curl localhost:8000/games/current_score/21
% 100