Add the List-Unsubscribe Header

Per https://support.google.com/mail/answer/81126?hl=en gmail
really likes it if we provide a List-Unsubscribe header. This
may help us for long term deliverability.

Change-Id: I07a100349244169cbc7d848ba58a5b2876bd34c7
Merge "Add the List-Unsubscribe Header"