fix test after mailing_list_enabled -> mailing_list_mode change
move initial upvote of submitted story to Story model
remove story downvoting, add story hiding

stories should either be reported for spam (coming later), upvoted,
or left alone rather than being downvoted for being uninteresting.
since users don't like leaving uninteresting things alone, they can
now hide stories from their view without affecting the story's
score.

hiding is implemented as a Vote with its vote set to 0 and the
reason set to "H"

add a /hidden url which shows all of a user's hidden stories

while i'm here, simplify Vote guts and add some tests to make sure
all the flip-flopping stuff works right