Support a Downs Only download option

[jpd236]

Provide a way for people to download puzzles with just the Down clues for an extra challenge.

Kotwords has a DownsOnly helper class for this - we mainly just need to expose it in a reasonable way in the UI.

Another detail is that the Kotwords DownsOnly helper tries to be clever by sometimes making an Acrosses Only puzzle instead when it notices the down clues have the longest answers (likely to be theme answers). I think that's a helpful mode in the spirit of the challenge, but it'd probably be good to offer it as an option while also having a strict Downs-Only mode instead.