Make turn headers into clickable links

Like line numbers in github diffs: you can click on a turn header, and your browser's location bar becomes a link you can paste to someone else. For example, forum topics like [Best Dominion Moments 2014](http://forum.dominionstrategy.com/index.php?topic=10127.msg344939#msg344939) would be improved by being able to link to a specific turn.

This one's a little harder to test live than the last change was, since it does stuff at pageload, and I can't modify your js until after the page has loaded. So while it does seem to work, I'm not 100% sure it will work under real conditions.