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.
Hmm. OK, so the behavior is definitely what I described earlier: there are multiple zombie tabs at a time, and this code kills them all:

function(tab) {
          try {
            chrome.tabs.remove(cesp.openTabId);
          } catch (err) { }
          cesp.openTabId = tab.id;
        });

But you're right, cesp.openTabId is only set to one value at a time! How is it killing all of them? I think because cesp.openTabId is evaluated when the function is defined, not when it's run. Ha. Confusing right.