Improve "Last Comment" Feature

This PR reduces the number of queries needed to build the home page by performing the score-based sort prior to fetching comment count and last comment time. It's a big difference, from 1803 queries to 63.

It also adds a query string to the comments link, which allows the browser to show the link as unvisited if there are new comments, addressing #200 (credit to http://www.joelonsoftware.com/articles/BuildingCommunitieswithSo.html).

Lastly, it adds the same functionality to the Issues page, and fixes a minor bug where the issue title was linking to the API instead of the regular Github site (#242).