Add level and message to sitebase (issue 306)

I don't really consider it a hack, but I don't really have a strong opinion about which way it should be, so I made the requested changes. I think I got all the calls to `render('index.html', ...)` (there were very few), and any actual `redirects` should use the `MainHandler`, which I also changed, so I can't think of any other places where changes would be necessary.