Add Downvoting to griply featurebase

I like being able to downvote feature suggestions, since plenty of features clearly aren’t liked by the community but can’t be downvoted. With enough downvotes, a suggestion should be deleted or scheduled for deletion.

Right now, features just pile up, and the only way forward is upvotes, so even bad features look good because they can only collect upvotes. We need downvoting to push the trash down and let the good stuff rise.

Also, please disable the auto-upvote when I leave a comment. Commenting doesn’t always mean I like the feature; sometimes I’m just commenting because I can’t downvote.