Add List-Unsubscribe to forum email header

The rational for this is outlined here and in many other places around email best practice:

http://www.list-unsubscribe.com/

ie if a user doesn't want to receive emails it should be very clear and simple to unsubscribe or else they may be more likely to hit the spam button and reduce the reputation of the whole system.

Many email clients such as gmail surface unsubscribe links in the email title bar if the correct headers are set and other conditions (like the sending server must DKIM sign the email and its reputation must be high). Additionally if the user hits the spam button they are instead asked if they want to unsubscribe instead, saving a potential reputation hit.

The forum emails in particular already have unsubscribe links inside the email body, but they don't set the correct headers.