Ability to not record certain stats

[flip-rossi]

User story

As a user, when logging a game, I sometimes don't track the match duration or the final player scores, but still want to, for example, register their positions (1st, 2nd, etc) and not have it affect the average stats for the game or players.

AC

• Allow to set "undefined" values that don't affect averages for:
  • match time;
  • individual player scores.
• Still allow player positionings to be recorded even with missing scores.

[mkieres]

hey @flip-rossi, thanks for capturing this. There's a similar issues #218, which I think is kind of a subset of this. I'm linking it here so when working on it's not missed.