Preliminary kick player support.

Only available to staff users. Not publicized.

Potentially enhancements:

  1. switch to using url/kwargs, i.e. /game/24/JohnSmith instead of /game/24/exit?player_name=JohnSmith 
  2. add kick buttons to score table (for people who can kick users)
  3. make the user who created a game the "owner", allow owners to kick players too (when owner exits, random player becomes owner unless owner reassigns ownership)