Upvote / Downvote subjects

[hialvaro]

Over time the subjects will accumulate and the older ones will no longer be useful. Or maybe some users will add useless/wrong subjects. It would be nice if users could vote on the subjects according to their validity/usefulness, showing the best ones at the top of the search, Reddit style.

Also, subjects with more than X down-votes could be automatically removed keeping a minimum of useful subjects and allowing users to self-moderate and maintain.