New feature: blank cards

Add support for blank white cards, that allow the player to enter their
own answer.  (Game host can choose how many to include in the deck; the
default is none.)
Missed edit
Merge latest upstream changes.
Increase max number of blank cards to 30.  (10 still seems like a reasonably comfortable number for most games, but if you're playing with a lot of decks enabled you might want a few more.)
Merge branch 'upstream' into blanks
Bugfix: assign unique (negative) ids to blank cards; previously the wrong card could win if multiple blanks were being judged in the same round.
Merge upstream changes.
Fix formatting.
Fix layout derp from merge.
Merge branch 'blanks' of https://github.com/uecasm/PretendYoureXyzzy into uecasm-blanks
Looks like cah.constants.js wasn't updated for BLANKS_LIMIT.