Bug 961637 - [email/UI] Quick advance regressed envelope display

The fix was two-fold:

* `clearDom` was a bit aggressive with the innerHTML clearing, just the peep bubbles needed to be removed, and make sure addHeaderEmails removes any previously collapsed state on a peep section if it means for it to be displayed.
* `postInsert` is no longer the right conceptual hook to use to display the message, onCurrentMessage is, so just moved the postInsert work into onCurrentMessage, as the other work done in onCurrentMessage needs to be done for each message displayed anyway. So, no more doubling of the peeps.

I did not attach a test as I feel we are still stabilizing the tests. I would prefer land a fix vs. creating more test code that may cause intermittents, and this is a very noticeable regression that impacts people dogfooding on master.