Add adminstrator privileges

[mhthies]

Only allow group administrators to

• set game options: synchronous mode, rounds, game mode (when Add "continuation story" mode #25 or Add "blind poem" mode #30 are implemented), display of names
• set language
• start game
• stop game
• stop game immediately

[mhthies]

This should be an optional feature: Any group adminstrator should be able to enable a restricted mode, in which only group adminstrators can change game settings and control the game.