Kill zombie windows

As described in #52, we have a zombie window problem:

1. Open a survey from a notification
2. Close the survey without submitting a form
3. Open another survey from the notification
4. The old survey that you closed will re-open (!!!)

This adds a hack to re-close all of the zombies as soon as they reopen.
Incorrect. Neither hitting the "x" nor doing chrome.tabs.remove kills the zombie tab. If I open a third survey, the previous 2 windows both try to open.