Add vote count field to NewsItem, Issue and Comment models.

This pull request increments a `vote_count` field on NewsItems, Comments and Issues as votes are saved. The idea is to eliminate the need to calculate votes for these items when calculating scores which should greatly improve the performance problems indicated in issue #328 while retaining the ability to dynamically calculate scores.

Note that this is only the first step in this proposed solution, and on it's own only begins to aggregate the vote counts for new items. Should we decide to fully implement this we would next need to calculate the vote counts for all existing items and only then could we refine the selection/scoring process to use these new values (as stated in the comments for Issue #328)