One vote per article per session

All upvoting now checks for a session variable specific to the link in
question. If such a variable exists, it indicates the user has already
voted (in that session) and so does not add a vote to the link. If no
session variable exists for that link, then the upvote is counted and a
session variable is set. This is not a perfect system -- you can vote
multiple times from multiple devices, by restarting your session, etc.
But it's more than adequate for now.