Feature: blank cards

This adds a new feature: the game host may optionally add 0-10 blank cards to the white deck, in addition to the standard card sets chosen.  (The default is 0.)

When a blank card is chosen by a player, it allows them to enter their own card text, which will then be visible to all other players as normal.  (Card text is HTML escaped for safety reasons, but otherwise it's free input.)

Player-entered card text is discarded when the blank card enters the discard pile, so if the white deck is reshuffled any previously used blank cards will return as fresh blank cards.