[logviewer] Disable autoscroll on new log when user scroll up

[mostynb]

I'm trying out buildbot based on the the tutorial on buildbot.net, using buildbot 0.9.6. I added a builder which has a reasonably long running time (several minutes), and started a build. Then when I go to view the build logs, if I scroll the view to a particular section of the log that I want to read, the view automatically jumps down to the bottom periodically, which can be quite frustrating.

Expected result: if the user scrolls away from the bottom of the build logs while the build is in progress, don't automatically scroll the logs (perhaps until the user scrolls back to the bottom).