A story downvote is considered a flag, just meaning the story has
problems and not necessarily that the user wants to ignore it. By
moving hiding out of Vote and into a new HiddenStory model, a user
can now both downvote/flag and hide separately, or just one or the
other.
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