Added browser warning message

@nicolashery can you look at this. I need to handle logout in a good way, i.e. when you logout it resets the browser warning overlay to show, i tried a few things but ended up not doing that part because all the solutions seemed hacked. You know the code base better than me, can you suggest a nice way to deal with it. Also @brandonarbiter or @HowardLook maybe you can say how important this is. The scenario is it gets used in a public machine and once someone dismises the message it will never be shown again in that machine regardless of who uses it. A solution is to show the message after login rather than at loading.