Adds both the List-Unsubscribe (RFC2369) and List-Unsubscribe-Post (RFC8058)
headers to push notification emails, which together should:
* Show an "Unsubscribe" link in the MUA UI when viewing Synapse notification emails.
* Enable "one-click" unsubscribe (the user never leaves their MUA, which automatically
makes a POST request to the specified endpoint).
