Prettification and a small bug fix.

Changes:

* Various style improvements. The chat box and buttons now looks a bit better and the time for chat messages is aligned to the right. Note that you can now see all the empty username boxes. Instead of thinking up a fix, I'll just let let it stay as a reminder that the way we're handling the chat box (inserting text into prewritten html and jumping the text around when another message comes in) is far from optimal.
* Use addEventListener for keydown/up. This is the recommended way of implementing events; using event attributes mixes script with markup and has some other drawbacks as well. All other event attributes should probably be moved to use addEventListener as well.
* Prevent the page from scrolling when pressing up or down keys. This also prevents cycling through the avatars unintentionally with up/down in Chrome but Firefox still does. Not sure why (maybe for accessibility?).