Call db.add_update when inserting and updating different single and group entities

[ogallagher]

• [x] update_user → add_update('people', 'username')
• [x] delete_user, recover_user, activate → add_update('people', 'username')
• [x] register → add_update('people', 'username')
• [x] play → add_update('plays', 'username,puzzle_id')
• [x] rate
  • [x] → add_update('ratings', 'username,puzzle_id')
  • [x] → add_update('puzzles', 'puzzle_id') Puzzle's average rating is also updated.
• [x] measure_difficulty
  • [x] → add_update('difficulties', 'username,puzzle_id')
  • [x] → add_update('puzzles', 'puzzle_id') Puzzle's average difficulty is also updated.
• [x] update_works → add_update('works', 'id') Works is a list of multiple; repeat for each work. Includes updates and deletes.
• contribute → add_update('works', 'id') When adding a work, we do not yet know it's unique id, but do not need to add an update version anyway because this is the work's oldest version anyway.