added auto-creation of BingoFields, extended view to create and use boards

* When there is no board, create one, redirect to its url, and set
  bingo_id as session-value
* When there is an bingo_id in the url, show that board, if it exists.
* When there is a bingo_id in session when visiting the url without
  bingo_id, redirect to this board.
* when a game is older than GAME_HARD_TIMEOUT minutes, or unused for
  GAME_SOFT_TIMEOUT minutes, start a new game, and assign new boards for
  even for users with a board_id in the session.